perlfaq/lib/perlglossary.pod

=encoding utf8

=head1 NAME

perlglossary - Perl Glossary

=head1 VERSION

version 5.20210520

=head1 DESCRIPTION

A glossary of terms (technical and otherwise) used in the Perl
documentation, derived from the Glossary of I<Programming
Perl>, Fourth Edition.  Words or phrases in bold are defined elsewhere in
this glossary.

Other useful sources include the Unicode Glossary L<http://unicode.org/glossary/>,
the Free On-Line Dictionary of Computing L<http://foldoc.org/>,
the Jargon File L<http://catb.org/~esr/jargon/>,
and Wikipedia L<http://www.wikipedia.org/>.

=head2 A

=over 4

=item accessor methods

A B<X<accessor methods, defined>X<methods, accessor>method> used to
indirectly inspect or update an B<object>’s state (its B<instance
variables>).

=item actual arguments

The B<X<actual arguments>X<arguments, actual>scalar values> that you supply
to a B<function> or B<subroutine> when you call it. For instance, when you
call C<power("puff")>, the string C<"puff"> is the actual argument. See also
B<argument> and B<formal arguments>.

=item address operator

Some X<address operator>languages work directly with the memory addresses of
values, but this can be like playing with fire. Perl provides a set of
asbestos gloves for handling all memory management. The closest to an
address operator in Perl is the backslash operator, but it gives you a
B<hard reference>, which is much safer than a memory address.

=item algorithm

A X<algorithms (term)>well-defined sequence of steps, explained clearly
enough that even a computer could do them.

=item alias

A X<aliases, defined>nickname for something, which behaves in all ways as
though you’d used the original name instead of the nickname. Temporary
aliases are implicitly created in the loop variable for C<foreach> loops, in
the C<$_> variable for C<map> or C<grep> operators, in C<$a> and C<$b>
during C<sort>’s comparison function, and in each element of C<@_> for the
B<actual arguments> of a subroutine call. Permanent aliases are explicitly
created in B<packages> by B<importing> symbols or by assignment to
B<typeglobs>. Lexically scoped aliases for package variables are explicitly
created by the C<our> declaration.

=item alphabetic

The X<alphabetic sort>sort of characters we put into words. In Unicode, this
is all letters including all ideographs and certain diacritics, letter
numbers like Roman numerals, and various combining marks.

=item alternatives

A X<alternative characters>list of possible choices from which you may
select only one, as in, “Would you like door A, B, or C?” Alternatives in
regular expressions are separated with a single vertical bar: C<|>.
Alternatives in normal Perl expressions are separated with a double vertical
bar: C<||>. Logical alternatives in B<Boolean> expressions are separated
with either C<||> or C<or>.

=item anonymous

Used to X<anonymous referents>X<referents, anonymous>describe a B<referent>
that is not directly accessible through a named B<variable>. Such a referent
must be indirectly accessible through at least one B<hard reference>. When
the last hard reference goes away, the anonymous referent is destroyed
without pity.

=item application

A X<applications (term)>bigger, fancier sort of B<program> with a fancier
name so people don’t realize they are using a program.

=item architecture

The kind of X<architecture>computer you’re working on, where one “kind of
computer” means all those computers sharing a compatible machine language.
Since Perl programs are (typically) simple text files, not executable
images, a Perl program is much less sensitive to the architecture it’s
running on than programs in other languages, such as C, that are B<compiled>
into machine code. See also B<platform> and B<operating system>.

=item argument

A X<arguments, defined>piece of data supplied to a B<program>,
B<subroutine>, B<function>, or B<method> to tell it what it’s supposed to
do. Also called a “parameter”.

=item ARGV

The name of the X<ARGV filehandle>array containing the B<argument> B<vector>
from the command line. If you use the empty C<E<lt>E<gt>> operator, C<ARGV>
is the name of both the B<filehandle> used to traverse the arguments and the
B<scalar> containing the name of the current input file.

=item arithmetical operator

A B<X<arithmetic operators, about>symbol> such as C<+> or C</> that tells
Perl to do the arithmetic you were supposed to learn in grade school.

=item array

An X<arrays, defined>ordered sequence of B<values>, stored such that you can
easily access any of the values using an I<integer subscript> that specifies
the value’s B<offset> in the sequence.

=item array context

An archaic X<array context>expression for what is more correctly referred to
as B<list context>.

=item Artistic License

The open X<Artistic License>source license that X<Wall, Larry>Larry Wall
created for Perl, maximizing Perl’s usefulness, availability, and
modifiability. The current version is 2. (L<http://www.opensource.org/licenses/artistic-license.php>).

=item ASCII

The X<ASCII (American Standard Code for Information Interchange)>X<American
Standard Code for Information Interchange (ASCII)>American Standard Code for
Information Interchange (a 7-bit character set adequate only for poorly
representing English text). Often used loosely to describe the lowest 128
values of the various ISO-8859-X character sets, a bunch of mutually
incompatible 8-bit codes best described as half ASCII. See also B<Unicode>.

=item assertion

A X<assertions (in regexes), defined>X<regular expressions, assertions
in>component of a B<regular expression> that must be true for the pattern to
match but does not necessarily match any characters itself. Often used
specifically to mean a B<zero-width> assertion.

=item assignment

An X<assignments, defined>B<operator> whose assigned mission in life is to
change the value of a B<variable>.

=item assignment operator

Either a X<assignment operators, about>regular B<assignment> or a compound
B<operator> composed of an ordinary assignment and some other operator, that
changes the value of a variable in place; that is, relative to its old
value. For example, C<$a += 2> adds C<2> to C<$a>.

=item associative array

See B<hash>. X<associative arrays>Please. The term associative array is the
old Perl 4 term for a B<hash>. Some languages call it a dictionary.

=item associativity

Determines X<associativity>whether you do the left B<operator> first or the
right B<operator> first when you have “A B<operator> B B<operator> C”, and
the two operators are of the same precedence. Operators like C<+> are left
associative, while operators like C<**> are right associative. See Camel
chapter 3, “Unary and Binary Operators” for a list of operators and their
associativity.

=item asynchronous

Said of X<asynchronous event processing>events or activities whose relative
temporal ordering is indeterminate because too many things are going on at
once. Hence, an asynchronous event is one you didn’t know when to expect.

=item atom

A B<regular X<atoms>expression> component potentially matching a
B<substring> containing one or more characters and treated as an indivisible
syntactic unit by any following B<quantifier>. (Contrast with an
B<assertion> that matches something of B<zero width> and may not be quantified.)

=item atomic operation

When X<atomic operation>Democritus gave the word “atom” to the indivisible
bits of matter, he meant literally something that could not be cut: I<ἀ->
(not) + I<-τομος> (cuttable). An atomic operation is an action that can’t be
interrupted, not one forbidden in a nuclear-free zone.

=item attribute

A new X<attribute feature>feature that allows the declaration of
B<variables> and B<subroutines> with modifiers, as in C<sub foo : locked
method>. Also another name for an B<instance variable> of an B<object>.

=item autogeneration

A X<autogeneration, about>feature of B<operator overloading> of B<objects>,
whereby the behavior of certain B<operators> can be reasonably deduced using
more fundamental operators. This assumes that the overloaded operators will
often have the same relationships as the regular operators. See Camel
chapter 13, “Overloading”.

=item autoincrement

To X<autoincrement (term)>add one to something automatically, hence the name
of the C<++> operator. To instead subtract one from something automatically
is known as an “autodecrement”.

=item autoload

To X<autoloading, defined>load on demand. (Also called “lazy” loading.)
Specifically, to call an C<AUTOLOAD> subroutine on behalf of an undefined
subroutine.

=item autosplit

To X<autosplit (term)>split a string automatically, as the I<–a> B<switch>
does when running under I<–p> or I<–n> in order to emulate B<awk>. (See also
the C<AutoSplit>X<AutoSplit module> module, which has nothing to do with the
C<–a> switch but a lot to do with autoloading.)

=item autovivification

A X<autovivification>Graeco-Roman word meaning “to bring oneself to life”.
In Perl, storage locations (B<lvalues>) spontaneously generate themselves as
needed, including the creation of any B<hard reference> values to point to
the next level of storage. The assignment C<$a[5][5][5][5][5] = "quintet">
potentially creates five scalar storage locations, plus four references (in
the first four scalar locations) pointing to four new anonymous arrays (to
hold the last four scalar locations). But the point of autovivification is
that you don’t have to worry about it.

=item AV

Short X<AV (array value)>X<array value (AV)>X<values, array>for “array
value”, which refers to one of Perl’s internal data types that holds an
B<array>. The C<AV> type is a subclass of B<SV>.

=item awk

Descriptive X<awk (editing term)>editing term—short for “awkward”. Also
coincidentally refers to a venerable text-processing language from which
Perl derived some of its high-level ideas.

=back

=head2 B

=over 4

=item backreference

A X<backreferences, about>X<references, backreferences>substring B<captured>
by a subpattern within unadorned parentheses in a B<regex>. Backslashed
decimal numbers (C<\1>, C<\2>, etc.) later in the same pattern refer back to
the corresponding subpattern in the current match. Outside the pattern, the
numbered variables (C<$1>, C<$2>, etc.) continue to refer to these same
values, as long as the pattern was the last successful match of the current
B<dynamic scope>.

=item backtracking

The X<backtracking>practice of saying, “If I had to do it all over, I’d do
it differently,” and then actually going back and doing it all over
differently. Mathematically speaking, it’s returning from an unsuccessful
recursion on a tree of possibilities. Perl backtracks when it attempts to
match patterns with a B<regular expression>, and its earlier attempts don’t
pan out. See the section “The Little Engine That /Couldn(n’t)” in Camel
chapter 5, “Pattern Matching”.

=item backward compatibility

Means X<backward compatibility, defined>you can still run your old program
because we didn’t break any of the features or bugs it was relying on.

=item bareword

A word X<barewords, about>sufficiently ambiguous to be deemed illegal under
C<use strict 'subs'>. In the absence of that stricture, a bareword is
treated as if quotes were around it.

=item base class

A X<base classes>X<classes, base>generic B<object> type; that is, a B<class>
from which other, more specific classes are derived genetically by
B<inheritance>. Also called aX<superclasses>X<classes, superclasses>
“superclass” by people who respect their ancestors.

=item big-endian

From X<big–endian, defined>X<endianness, big–endian>Swift: someone who
eats eggs big end first. Also used of computers that store the most
significant B<byte> of a word at a lower byte address than the least
significant byte. Often considered superior to little-endian machines. See
also B<little-endian>.

=item binary

Having X<binary (term)>to do with numbers represented in base 2. That means
there’s basically two numbers: 0 and 1. Also used to describe a file of
“nontext”, presumably because such a file makes full use of all the binary
bits in its bytes. With the advent of B<Unicode>, this distinction, already
suspect, loses even more of its meaning.

=item binary operator

An B<X<binary operators, about>operator> that takes two B<operands>.

=item bind

To X<bind (term)>assign a specific B<network address> to a B<socket>.

=item bit

An X<bits, defined>integer in the range from 0 to 1, inclusive. The smallest
possible unit of information storage. An eighth of a B<byte> or of a dollar.
(The term “Pieces of Eight” comes from being able to split the old Spanish
dollar into 8 bits, each of which still counted for money. That’s why a 25-
cent piece today is still “two bits”.)

=item bit shift

The X<bit–shift operators, defined>movement of bits left or right in a
computer word, which has the effect of multiplying or dividing by a
power of 2.

=item bit string

A X<bit string>sequence of B<bits> that is actually being thought of as a
sequence of bits, for once.

=item bless

In X<bless function, about>X<bless (term)>corporate life, to grant official
approval to a thing, as in, “The VP of Engineering has blessed our
WebCruncher project.” Similarly, in Perl, to grant official approval to a
B<referent> so that it can function as an B<object>, such as a WebCruncher
object. See the C<bless> function in Camel chapter 27, “Functions”.

=item block

What X<blocks, defined>a B<process> does when it has to wait for something:
“My process blocked waiting for the disk.” As an unrelated noun, it refers
to a large chunk of data, of a size that the B<operating system> likes to
deal with (normally a power of 2 such as 512 or 8192). Typically refers to
a chunk of data that’s coming from or going to a disk file.

=item BLOCK

A X<BLOCK construct, about>X<constructs, BLOCK>syntactic construct
consisting of a sequence of Perl B<statements> that is delimited by braces.
The C<if> and C<while> statements are defined in terms of I<C<BLOCK>>s, for
instance. Sometimes we also say “block” to mean a lexical scope; that is, a
sequence of statements that acts like a I<C<BLOCK>>, such as within an
C<eval> or a file, even though the statements aren’t delimited by braces.

=item block buffering

A X<block buffering>X<buffering, block>method of making input and output
efficient by passing one B<block> at a time. By default, Perl does block
buffering to disk files. See B<buffer> and B<command buffering>.

=item Boolean

A X<Boolean values>X<values, Boolean>value that is either B<true> or
B<false>.

=item Boolean context

A X<Boolean context, about>X<context, Boolean>special kind of B<scalar
context> used in conditionals to decide whether the B<scalar value> returned
by an expression is B<true> or B<false>. Does not evaluate as either a
string or a number. See B<context>.

=item breakpoint

A X<breakpoints, defined>spot in your program where you’ve told the debugger
to stop B<execution> so you can poke around and see whether anything is
wrong yet.

=item broadcast

To X<broadcast (networking term)>send a B<datagram> to multiple destinations
simultaneously.

=item BSD

A X<BSD (Berkeley Standard Distribution)>X<Berkeley Standard Distribution
(BSD)>psychoactive drug, popular in the ’80s, probably developed at UC
Berkeley or thereabouts. Similar in many ways to the prescription-only
medication called “System V”, but infinitely more useful. (Or, at least,
more fun.) The full chemical name is “Berkeley Standard Distribution”.

=item bucket

A X<buckets (term)>location in a B<hash table> containing (potentially)
multiple entries whose keys “hash” to the same hash value according to its
hash function. (As internal policy, you don’t have to worry about it unless
you’re into internals, or policy.)

=item buffer

A X<buffers, defined>temporary holding location for data. Data that are
B<Block buffering> means that the data is passed on to its destination
whenever the buffer is full. B<Line buffering> means that it’s passed on
whenever a complete line is received. B<Command buffering> means that it’s
passed every time you do a C<print> command (or equivalent). If your output
is unbuffered, the system processes it one byte at a time without the use of
a holding area. This can be rather inefficient.

=item built-in

A B<X<built–in functions, about>function> that is predefined in the
language. Even when hidden by B<overriding>, you can always get at a built-
in function by B<qualifying> its name with the C<CORE::> pseudopackage.

=item bundle

A X<bundles (term)>group of related modules on B<CPAN>. (Also sometimes
refers to a group of command-line switches grouped into one B<switch
cluster>.)

=item byte

A X<bytes (term)>piece of data worth eight B<bits> in most places.

=item bytecode

A pidgin-like lingo spoken among ’droids when they don’t wish to reveal
their orientation (see B<endian>). Named after some similar languages spoken
(for similar reasons) between compilers and interpreters in the late 20ᵗʰ
century. These languages are characterized by representing everything as a
nonarchitecture-dependent sequence of bytes.

=back

=head2 C

=over 4

=item C

A X<C language, about>language beloved by many for its inside-out B<type>
definitions, inscrutable B<precedence> rules, and heavy B<overloading> of
the function-call mechanism. (Well, actually, people first switched to C
because they found lowercase identifiers easier to read than upper.) Perl is
written in C, so it’s not surprising that Perl borrowed a few ideas from it.

=item cache

A X<cache (term)>data repository. Instead of computing expensive answers
several times, compute it once and save the result.

=item callback

A B<X<callbacks>handler> that you register with some other part of your
program in the hope that the other part of your program will B<trigger> your
handler when some event of interest transpires.

=item call by reference

An B<argument>-passing X<call by reference>X<references, call by reference
mechanism>mechanism in which the B<formal arguments> refer directly to the
B<actual arguments>, and the B<subroutine> can change the actual arguments
by changing the formal arguments. That is, the formal argument is an
B<alias> for the actual argument. See also B<call by value>.

=item call by value

An B<X<call by value>argument>-passing mechanism in which the B<formal
arguments> refer to a copy of the B<actual arguments>, and the
B<subroutine> cannot change the actual arguments by changing the formal
arguments. See also B<call by reference>.

=item canonical

Reduced X<canonical (term)>to a standard form to facilitate comparison.

=item capture variables

The X<capture variables>X<variables, capture>variables—such as C<$1> and
C<$2>, and C<%+> and C<%– >—that hold the text remembered in a pattern
match. See Camel chapter 5, “Pattern Matching”.

=item capturing

The X<capturing in pattern matching>X<subpatterns, capturing>X<pattern
matching, capturing in>use of parentheses around a B<subpattern> in a
B<regular expression> to store the matched B<substring> as a
B<backreference>. (Captured strings are also returned as a list in B<list
context>.) See Camel chapter 5, “Pattern Matching”.

=item cargo cult

Copying X<cargo cult>and pasting code without understanding it, while
superstitiously believing in its value. This term originated from
preindustrial cultures dealing with the detritus of explorers and colonizers
of technologically advanced cultures. See I<The Gods Must Be Crazy>.

=item case

A X<case (character)>X<characters, case considerations>property of certain
characters. Originally, typesetter stored capital letters in the upper of
two cases and small letters in the lower one. Unicode recognizes three
cases: B<lowercase> (B<character property> C<\p{lower}>), B<titlecase>
(C<\p{title}>), and B<uppercase> (C<\p{upper}>). A fourth casemapping called
B<foldcase> is not itself a distinct case, but it is used internally to
implement B<casefolding>. Not all letters have case, and some nonletters
have case.

=item casefolding

Comparing X<casefolding>or matching a string case-insensitively. In Perl, it
is implemented with the C</i> pattern modifier, the C<fc> function, and the
C<\F> double-quote translation escape.

=item casemapping

The X<casemapping>process of converting a string to one of the four Unicode
B<casemaps>; in Perl, it is implemented with the C<fc>, C<lc>, C<ucfirst>,
and C<uc> functions.

=item character

The X<characters, defined>smallest individual element of a string. Computers
store characters as integers, but Perl lets you operate on them as text. The
integer used to represent a particular character is called that character’s
B<codepoint>.

=item character class

A X<character classes, about>X<classes, character>square-bracketed list of
characters used in a B<regular expression> to indicate that any character
of the set may occur at a given point. Loosely, any predefined set of
characters so used.

=item character property

A X<character property>predefined B<character class> matchable by the C<\p>
or C<\P> B<metasymbol>. B<Unicode> defines hundreds of standard properties
for every possible codepoint, and Perl defines a few of its own, too.

=item circumfix operator

An X<circumfix operator>B<operator> that surrounds its B<operand>, like the
angle operator, or parentheses, or a hug.

=item class

A X<classes, defined>user-defined B<type>, implemented in Perl via a
B<package> that provides (either directly or by inheritance) B<methods>
(that is, B<subroutines>) to handle B<instances> of the class (its
B<objects>). See also B<inheritance>.

=item class method

A B<X<class methods>X<methods, class>method> whose B<invocant> is a
B<package> name, not an B<object> reference. A method associated with the
class as a whole. Also see B<instance method>.

=item client

In X<clients, defined>X<processes, client>networking, a B<process> that
initiates contact with a B<server> process in order to exchange data and
perhaps receive a service.

=item closure

An B<X<closure subroutines>X<subroutines, closure>anonymous> subroutine
that, when a reference to it is generated at runtime, keeps track of the
identities of externally visible B<lexical variables>, even after those
lexical variables have supposedly gone out of B<scope>. They’re called
“closures” because this sort of behavior gives mathematicians a sense of
closure.

=item cluster

A X<clusters, defined>X<subpatterns, cluster>parenthesized B<subpattern>
used to group parts of a B<regular expression> into a single B<atom>.

=item CODE

The X<CODE (ref function)>X<ref function, about>word returned by the C<ref>
function when you apply it to a reference to a subroutine. See also B<CV>.

=item code generator

A X<code generators, defined>system that writes code for you in a low-level
language, such as code to implement the backend of a compiler. See B<program
generator>.

=item codepoint

The X<codepoints, about>integer a computer uses to represent a given
character. ASCII codepoints are in the range 0 to 127; Unicode codepoints
are in the range 0 to 0x1F_FFFF; and Perl codepoints are in the range 0 to
2³²−1 or 0 to 2⁶⁴−1, depending on your native integer size. In Perl Culture,
sometimes called B<ordinals>.

=item code subpattern

A B<X<code subpatterns>X<subpatterns, code>regular expression> subpattern
whose real purpose is to execute some Perl code—for example, the C<(?{...})>
and C<(??{...})> subpatterns.

=item collating sequence

The X<collating sequence>X<collating sequence>order into which B<characters>
sort. This is used by B<string> comparison routines to decide, for example,
where in this glossary to put “collating sequence”.

=item co-maintainer

A X<co–maintainers>person with permissions to index a B<namespace> in
B<PAUSE>. Anyone can upload any namespace, but only primary and
co-maintainers get their contributions indexed.

=item combining character

Any X<combining characters>X<characters, combining>character with the
General Category of Combining Mark (C<\p{GC=M}>), which may be spacing or
nonspacing. Some are even invisible. A sequence of combining characters
following a grapheme base character together make up a single user-visible
character called a B<grapheme>. Most but not all diacritics are combining
characters, and vice versa.

=item command

In B<shell> X<commands, defined>programming, the syntactic combination of a
program name and its arguments. More loosely, anything you type to a shell
(a command interpreter) that starts it doing something. Even more loosely, a
Perl B<statement>, which might start with a B<label> and typically ends with
a semicolon.

=item command buffering

A X<command buffering>X<buffering, command>mechanism in Perl that lets you
store up the output of each Perl B<command> and then flush it out as a
single request to the B<operating system>. It’s enabled by setting the C<$|>
(C<$AUTOFLUSH>) variable to a true value. It’s used when you don’t want data
sitting around, not going where it’s supposed to, which may happen because
the default on a B<file> or B<pipe> is to use B<block buffering>.

=item command-line arguments

The X<command–line arguments>B<X<arguments, command–line>values> you supply
along with a program name when you tell a B<shell> to execute a B<command>.
These values are passed to a Perl program through C<@ARGV>.

=item command name

The X<command names>name of the program currently executing, as typed on the
command line. In C, the B<command> name is passed to the program as the
first command-line argument. In Perl, it comes in separately as C<$0>.

=item comment

A X<comments, defined>remark that doesn’t affect the meaning of the program.
In Perl, a comment is introduced by a C<#> character and continues to the
end of the line.

=item compilation unit

The X<compilation units>B<file> (or B<string>, in the case of C<eval>) that
is currently being B<compiled>.

=item compile

The process of turning source code into a machine-usable form. See B<compile
phase>.

=item compile phase

Any X<compile phase, defined>time before Perl starts running your main
program. See also B<run phase>. Compile phase is mostly spent in B<compile
time>, but may also be spent in B<runtime> when C<BEGIN> blocks, C<use> or
C<no> declarations, or constant subexpressions are being evaluated. The
startup and import code of any C<use> declaration is also run during
compile phase.

=item compiler

Strictly X<compilers and compiling, about>speaking, a program that munches
up another program and spits out yet another file containing the program in
a “more executable” form, typically containing native machine instructions.
The I<perl> program is not a compiler by this definition, but it does
contain a kind of compiler that takes a program and turns it into a more
executable form (B<syntax trees>) within the I<perl> process itself, which
the B<interpreter> then interprets. There are, however, extension B<modules>
to get Perl to act more like a “real” compiler. See Camel chapter 16,
“Compiling”.

=item compile time

The X<compile time, defined>time when Perl is trying to make sense of your
code, as opposed to when it thinks it knows what your code means and is
merely trying to do what it thinks your code says to do, which is B<runtime>.

=item composer

A “constructor” X<composers, about>for a B<referent> that isn’t really an
B<object>, like an anonymous array or a hash (or a sonata, for that matter).
For example, a pair of braces acts as a composer for a hash, and a pair of
brackets acts as a composer for an array. See the section “Creating
References” in Camel chapter 8, “References”.

=item concatenation

The X<concatenating strings>X<strings, concatenating>process of gluing one
cat’s nose to another cat’s tail. Also a similar operation on two
B<strings>.

=item conditional

SomethingX<conditional (term)> “iffy”. See B<Boolean context>.

=item connection

In X<connections (term)>telephony, the temporary electrical circuit between
the caller’s and the callee’s phone. In networking, the same kind of
temporary circuit between a B<client> and a B<server>.

=item construct

As a X<constructs, defined>noun, a piece of syntax made up of smaller
pieces. As a transitive verb, to create an B<object> using a B<constructor>.

=item constructor

AnyX<constructors, defined> B<class method>, B<instance>, or B<subroutine>
that composes, initializes, blesses, and returns an B<object>. Sometimes we
use the term loosely to mean a B<composer>.

=item context

The X<context, about>surroundings or environment. The context given by the
surrounding code determines what kind of data a particular B<expression> is
expected to return. The three primary contexts are B<list context>,
B<scalar>, and B<void context>. Scalar context is sometimes subdivided into
B<Boolean context>, B<numeric context>, B<string context>, and B<void
context>. There’s also a “don’t care” context (which is dealt with in Camel
chapter 2, “Bits and Pieces”, if you care).

=item continuation

The X<continuation lines>treatment of more than one physical B<line> as a
single logical line. B<Makefile> lines are continued by putting a backslash
before the B<newline>. Mail headers, as defined by X<RFC 822>RFC 822, are
continued by putting a space or tab I<after> the newline. In general, lines
in Perl do not need any form of continuation mark, because B<whitespace>
(including newlines) is gleefully ignored. Usually.

=item core dump

The X<core dump>corpse of a B<process>, in the form of a file left in the
B<working directory> of the process, usually as a result of certain kinds
of fatal errors.

=item CPAN

The X<Comprehensive Perl Archive Network>X<CPAN (Comprehensive Perl Archive
Network), about>Comprehensive Perl Archive Network. (See the Camel Preface
and Camel chapter 19, “CPAN” for details.)

=item C preprocessor

The X<C preprocessor>typical C compiler’s first pass, which processes lines
beginning with C<#> for conditional compilation and macro definition, and
does various manipulations of the program text based on the current
definitions. Also known as I<cpp>(1).

=item cracker

Someone X<crackers>who breaks security on computer systems. A cracker may
be a true B<hacker> or only a B<script kiddie>.

=item currently selected output channel

The X<currently selected output channel>last B<filehandle> that was
designated with C<select(FILEHANDLE)>; C<STDOUT>, if no filehandle has
been selected.

=item current package

The B<package> X<current package>in which the current statement is
B<compiled>. Scan backward in the text of your program through the current
B<lexical scope> or any enclosing lexical scopes until you find a package
declaration. That’s your current package name.

=item current working directory

SeeX<current working directory> B<working directory>.

=item CV

In academia, a curriculum vitæ, a fancy kind of résumé. In Perl, an X<CV
(code value)>X<code value (CV)>internal “code value” typedef holding a
B<subroutine>. The C<CV> type is a subclass of B<SV>.

=back

=head2 D

=over 4

=item dangling statement

A bare, single B<X<dangling statements>X<statements, dangling>statement>,
without any braces, hanging off an C<if> or C<while> conditional. C allows
them. Perl doesn’t.

=item datagram

A packet of X<datagrams, defined>data, such as a B<UDP> message, that (from
the viewpoint of the programs involved) can be sent independently over the
network. (In fact, all packets are sent independently at the B<IP> level,
but B<stream> protocols such as B<TCP> hide this from your program.)

=item data structure

How your X<data structures, defined>various pieces of data relate to each
other and what shape they make when you put them all together, as in a
rectangular table or a triangular tree.

=item data type

A set of X<data types, defined>possible values, together with all the
operations that know how to deal with those values. For example, a numeric
data type has a certain set of numbers that you can work with, as well as
various mathematical operations that you can do on the numbers, but would
make little sense on, say, a string such as C<"Kilroy">. Strings have their
own operations, such as B<concatenation>. Compound types made of a number of
smaller pieces generally have operations to compose and decompose them, and
perhaps to rearrange them. B<Objects> that model things in the real world
often have operations that correspond to real activities. For instance, if
you model an elevator, your elevator object might have an C<open_door>
B<method>.

=item DBM

Stands for X<DBM (Database Management) routines>X<Database Management (DBM)
routines>“Database Management” routines, a set of routines that emulate an
B<associative array> using disk files. The routines use a dynamic hashing
scheme to locate any entry with only two disk accesses. DBM files allow a
Perl program to keep a persistent B<hash> across multiple invocations. You
can C<tie> your hash variables to various DBM implementations.

=item declaration

An B<assertion> X<declarations, defined>that states something exists and
perhaps describes what it’s like, without giving any commitment as to how
or where you’ll use it. A declaration is like the part of your recipe that
says, “two cups flour, one large egg, four or five tadpoles…” See
B<statement> for its opposite. Note that some declarations also function
as statements. Subroutine declarations also act as definitions if a body
is supplied.

=item declarator

Something X<declarators>that tells your program what sort of variable
you’d like. Perl doesn’t require you to declare variables, but you can use
C<my>, C<our>, or C<state> to denote that you want something other than
the default.

=item decrement

To X<decrementing values>X<values, decrementing>subtract a value from a
variable, as in “decrement C<$x>” (meaning to remove 1 from its value) or
“decrement C<$x> by 3”.

=item default

A B<value> X<default values>X<values, default>chosen for you if you don’t
supply a value of your own.

=item defined

Having a X<defined (term)>meaning. Perl thinks that some of the things
people try to do are devoid of meaning; in particular, making use of
variables that have never been given a B<value> and performing certain
operations on data that isn’t there. For example, if you try to read data
past the end of a file, Perl will hand you back an undefined value. See also
B<false> and the C<defined> entry in Camel chapter 27, “Functions”.

=item delimiter

A B<character> X<delimiters (term)>or B<string> that sets bounds to an
arbitrarily sized textual object, not to be confused with a B<separator> or
B<terminator>. “To delimit” really just means “to surround” or “to enclose”
(like these parentheses are doing).

=item dereference

A fancy X<dereference (term)>X<references, dereference>computer science term
meaning “to follow a B<reference> to what it points to”. The “de” part of it
refers to the fact that you’re taking away one level of B<indirection>.

=item derived class

A B<class> that X<derived classes>X<classes, derived>X<subclasses>X<classes,
subclasses>defines some of its B<methods> in terms of a more generic class,
called a B<base class>. Note that classes aren’t classified exclusively into
base classes or derived classes: a class can function as both a derived
class and a base class simultaneously, which is kind of classy.

=item descriptor

See B<file descriptor>.

=item destroy

To deallocate the X<destroy (term)>memory of a B<referent> (first triggering
its C<DESTROY> method, if it has one).

=item destructor

A special B<method> X<destructor method>X<methods, destructor>that is called
when an B<object> is thinking about B<destroying> itself. A Perl program’s
C<DESTROY> method doesn’t do the actual destruction; Perl just B<triggers>
the method in case the B<class> wants to do any associated cleanup.

=item device

A whiz-bang X<devices (term)>hardware gizmo (like a disk or tape drive or a
modem or a joystick or a mouse) attached to your computer, which the
B<operating system> tries to make look like a B<file> (or a bunch of files).
Under Unix, these fake files tend to live in the I</dev> directory.

=item directive

A B<pod> X<directives, defined>directive. See Camel chapter 23, “Plain Old
Documentation”.

=item directory

A special X<directories, defined>file that contains other files. Some
B<operating systems> call these “folders”, “drawers”, “catalogues”, or
“catalogs”.

=item directory handle

A name X<directory handle>that represents a particular instance of opening a
directory to read it, until you close it. See the C<opendir> function.

=item discipline

Some X<discipline (I/O layer)>people need this and some people avoid it.
For Perl, it’s an old way to say B<I/O layer>.

=item dispatch

To send X<dispatching>something to its correct destination. Often used
metaphorically to indicate a transfer of programmatic control to a
destination selected algorithmically, often by lookup in a table of function
B<references> or, in the case of object B<methods>, by traversing the
inheritance tree looking for the most specific definition for the method.

=item distribution

A standard, X<distributions, defined>bundled release of a system of
software. The default usage implies source code is included. If that is not
the case, it will be called a “binary-only” distribution.

=item dual-lived

Some X<dual–lived modules>X<modules, dual–lived>modules live both in the
B<Standard Library> and on B<CPAN>. These modules might be developed on two
tracks as people modify either version. The trend currently is to untangle
these situations.

=item dweomer

An enchantment, illusion, X<dweomer>phantasm, or jugglery. Said when Perl’s
magical B<dwimmer> effects don’t do what you expect, but rather seem to be
the product of arcane I<dweomercraft>, sorcery, or wonder working. [From
Middle English.]

=item dwimmer

DWIM X<DWIM (Do What I Mean) principle>X<Do What I Mean (DWIM) principle>is
an acronym for X<dwimming>“Do What I Mean”, the principle that something
should just do what you want it to do without an undue amount of fuss. A bit
of code that does “dwimming” is a “dwimmer”. Dwimming can require a great
deal of behind-the-scenes magic, which (if it doesn’t stay properly behind
the scenes) is called a B<dweomer> instead.

=item dynamic scoping

Dynamic X<dynamic scope>X<scopes, dynamic>scoping works over a B<dynamic
scope>, making variables visible throughout the rest of the B<block> in
which they are first used and in any B<subroutines> that are called by the
rest of the block. Dynamically scoped variables can have their values
temporarily changed (and implicitly restored later) by a C<local> operator.
(Compare B<lexical scoping>.) Used more loosely to mean how a subroutine
that is in the middle of calling another subroutine “contains” that
subroutine at B<runtime>.

=back

=head2 E

=over 4

=item eclectic

Derived X<eclectic (term)>from many sources. Some would say I<too> many.

=item element

A basic X<elements, about>building block. When you’re talking about an
B<array>, it’s one of the items that make up the array.

=item embedding

When X<embedding (term)>something is contained in something else,
particularly when that might be considered surprising: “I’ve embedded a
complete Perl interpreter in my editor!”

=item empty subclass test

The notion X<empty subclass test>that an empty B<derived class> should
behave exactly like its B<base class>.

=item encapsulation

The veil of X<encapsulation (term)>abstraction separating the B<interface>
from the B<implementation> (whether enforced or not), which mandates that
all access to an B<object>’s state be through B<methods> alone.

=item endian

See B<little-endian> and B<big-endian>.

=item en passant

When you X<en passant (term)>change a B<value> as it is being copied. [From
French “in passing”, as in the exotic pawn-capturing maneuver in chess.]

=item environment

The collectiveX<environment (term)> set of B<environment variables> your
B<process> inherits from its parent. Accessed via C<%ENV>.

=item environment variable

A mechanism X<environment variables>X<variables, environment>X<environment
variables>by which some high-level agent such as a user can pass its
preferences down to its future offspring (child B<processes>, grandchild
processes, great-grandchild processes, and so on). Each environment
variable is a B<key>/B<value> pair, like one entry in a B<hash>.

=item EOF

End of File. X<End of File (EOF)>X<EOF (End of File)>Sometimes used
metaphorically as the terminating string of a B<here document>.

=item errno

The X<errno (error number)>X<error number (errno)>error number returned by a
B<syscall> when it fails. Perl refers to the error by the name C<$!> (or
C<$OS_ERROR> if you use the English module).

=item error

See B<exception> or B<fatal error>.

=item escape sequence

See B<metasymbol>.

=item exception

A fancy term for an error. See B<fatal error>.

=item exception handling

The X<exception handling, defined>way a program responds to an error. The
exception-handling mechanism in Perl is the C<eval> operator.

=item exec

To X<exec function>throw away the current B<process>’s program and replace
it with another, without exiting the process or relinquishing any resources
held (apart from the old memory image).

=item executable file

A B<file> X<executable files>X<files, executable>that is specially marked to
tell the B<operating system> that it’s okay to run this file as a program.
Usually shortened to “executable”.

=item execute

To run X<execute (term)>a B<program> or B<subroutine>. (Has nothing to do
with the C<kill> built-in, unless you’re trying to run a B<signal handler>.)

=item execute bit

The X<execute bit>special mark that tells the operating system it can run
this program. There are actually three execute bits under Unix, and which
bit gets used depends on whether you own the file singularly, collectively,
or not at all.

=item exit status

See B<status>.

=item exploit

Used X<exploits, security>as a noun in this case, this refers to a known way
to compromise a program to get it to do something the author didn’t intend.
Your task is to write unexploitable programs.

=item export

To make X<exporting, defined>symbols from a B<module> available for
B<import> by other modules.

=item expression

Anything X<expressions, defined>X<expressions>you can legally say in a spot
where a B<value> is required. Typically composed of B<literals>,
B<variables>, B<operators>, B<functions>, and B<subroutine> calls, not
necessarily in that order.

=item extension

A Perl module X<extensions, defined>that also pulls in B<compiled> C or C++
code. More generally, any experimental option that can be B<compiled> into
Perl, such as multithreading.

=back

=head2 F

=over 4

=item false

In Perl, any value X<false values>X<values, false>that would look like C<"">
or C<"0"> if evaluated in a string context. Since undefined values evaluate
to C<"">, all undefined values are false, but not all false values are
undefined.

=item FAQ

Frequently Asked QuestionX<FAQ (Frequently Asked
Question)>X<Frequently Asked Question (FAQ)> (although not necessarily
frequently answered, especially if the answer appears in the Perl FAQ
shipped standard with Perl).

=item fatal error

An uncaught B<exception>, X<fatal errors>which causes termination of the
B<process> after printing a message on your B<standard error> stream. Errors
that happen inside an C<eval> are not fatal. Instead, the C<eval> terminates
after placing the exception message in the C<$@> (C<$EVAL_ERROR>) variable.
You can try to provoke a fatal error with the C<die> operator (known as
throwing or raising an exception), but this may be caught by a dynamically
enclosing C<eval>. If not caught, the C<die> becomes a fatal error.

=item feeping creaturism

A spoonerism X<feeping creaturism>X<creeping featurism>of “creeping
featurism”, noting the biological urge to add just one more feature to
a program.

=item field

A single X<fields (term)>piece of numeric or string data that is part of a
longer B<string>, B<record>, or B<line>. Variable-width fields are usually
split up by B<separators> (so use C<split> to extract the fields), while
fixed-width fields are usually at fixed positions (so use C<unpack>).
B<Instance variables> are also known as “fields”.

=item FIFO

First In, First Out.X<First In, First Out (FIFO)>X<FIFO (First In, First
Out)> See also B<LIFO>. Also a nickname for a B<named pipe>.

=item file

A named X<files, defined>collection of data, usually stored on disk in a
B<directory> in a B<filesystem>. Roughly like a document, if you’re into
office metaphors. In modern filesystems, you can actually give a file more
than one name. Some files have special properties, like directories and
devices.

=item file descriptor

The little X<file descriptors>X<descriptors, file>number the B<operating
system> uses to keep track of which opened B<file> you’re talking about.
Perl hides the file descriptor inside a B<standard I/O> stream and then
attaches the stream to a B<filehandle>.

=item fileglob

A “wildcard” X<fileglobs>match on B<filenames>. See the C<glob> function.

=item filehandle

An identifier X<filehandles, about>(not necessarily related to the real
name of a file) that represents a particular instance of opening a file,
until you close it. If you’re going to open and close several different
files in succession, it’s fine to open each of them with the same
filehandle, so you don’t have to write out separate code to process each
file.

=item filename

One name for a X<filenames, about>file. This name is listed in a
B<directory>. You can use it in an C<open> to tell the B<operating system>
exactly which file you want to open, and associate the file with a
B<filehandle>, which will carry the subsequent identity of that file in
your program, until you close it.

=item filesystem

A set X<filesystems, defined>of B<directories> and B<files> residing on a
partition of the disk. Sometimes known as a “partition”. You can change the
file’s name or even move a file around from directory to directory within a
filesystem without actually moving the file itself, at least under Unix.

=item file test operator

A built-in X<file test operators, about>unary operator that you use to
determine whether something is B<true> about a file, such as C<–o
$filename> to test whether you’re the owner of the file.

=item filter

A X<filters, defined>program designed to take a B<stream> of input and
transform it into a stream of output.

=item first-come

The X<first–come permissions>X<permissions, first–come>first B<PAUSE>
author to upload a B<namespace> automatically becomes the B<primary
maintainer> for that namespace. The “first come” permissions distinguish a
B<primary maintainer> who was assigned that role from one who received it
automatically.

=item flag

We X<flags (term)>tend to avoid this term because it means so many things.
It may mean a command-line B<switch> that takes no argument itself (such as
Perl’s C<–n> and C<–p> flags) or, less frequently, a single-bit indicator
(such as the C<O_CREAT> and C<O_EXCL> flags used in C<sysopen>). Sometimes
informally used to refer to certain regex modifiers.

=item floating point

A X<floating point methods>X<methods, floating point>method of storing
numbers in “scientific notation”, such that the precision of the number is
independent of its magnitude (the decimal point “floats”). Perl does its
numeric work with floating-point numbers (sometimes called “floats”) when
it can’t get away with using B<integers>. Floating-point numbers are mere
approximations of real numbers.

=item flush

The act of X<flushing buffers>X<buffers, flushing>emptying a B<buffer>,
often before it’s full.

=item FMTEYEWTK

Far More Than Everything You Ever Wanted To KnowX<FMTEYEWTK acronym>. An
exhaustive treatise on one narrow topic, something of a super-B<FAQ>. See
Tom for far more.

=item foldcase

The casemap X<foldcase (term)>used in Unicode when comparing or matching
without regard to case. Comparing lower-, title-, or uppercase are all
unreliable due to Unicode’s complex, one-to-many case mappings. Foldcase is
a B<lowercase> variant (using a partially decomposed B<normalization> form
for certain codepoints) created specifically to resolve this.

=item fork

To create a X<forking processes>X<processes, forking>child B<process>
identical to the parent process at its moment of conception, at least until
it gets ideas of its own. A thread with protected memory.

=item formal arguments

The X<formal arguments>X<arguments, formal>generic names by which a
B<subroutine> knows its B<arguments>. In many languages, formal arguments
are always given individual names; in Perl, the formal arguments are just
the elements of an array. The formal arguments to a Perl program are
C<$ARGV[0]>, C<$ARGV[1]>, and so on. Similarly, the formal arguments to a
Perl subroutine are C<$_[0]>, C<$_[1]>, and so on. You may give the
arguments individual names by assigning the values to a C<my> list. See
also B<actual arguments>.

=item format

A X<formats, defined>specification of how many spaces and digits and things
to put somewhere so that whatever you’re printing comes out nice and
pretty.

=item freely available

Means X<freely available (term)>you don’t have to pay money to get it, but
the copyright on it may still belong to someone else (like Larry).

=item freely redistributable

Means X<freely redistributable (term)>you’re not in legal trouble if you
give a bootleg copy of it to your friends and we find out about it. In
fact, we’d rather you gave a copy to all your friends.

=item freeware

Historically, X<freeware (term)>any software that you give away,
particularly if you make the source code available as well. Now often
called B<open source software>. Recently there has been a trend to use the
term in contradistinction to B<open source software>, to refer only to free
software released under the X<Free Software Foundation>Free Software
Foundation’s GPL (General Public License), but this is difficult to justify
etymologically.

=item function

Mathematically, X<functions, about>a mapping of each of a set of input
values to a particular output value. In computers, refers to a
B<subroutine> or B<operator> that returns a B<value>. It may or may not
have input values (called B<arguments>).

=item funny character

Someone X<funny characters>X<characters, funny>like Larry, or one of his
peculiar friends. Also refers to the strange prefixes that Perl requires as
noun markers on its variables.

=back

=head2 G

=over 4

=item garbage collection

A misnamed feature—X<garbage collection, defined>it should be called,
“expecting your mother to pick up after you”. Strictly speaking, Perl
doesn’t do this, but it relies on a reference-counting mechanism to keep
things tidy. However, we rarely speak strictly and will often refer to the
reference-counting scheme as a form of garbage collection. (If it’s any
comfort, when your interpreter exits, a “real” garbage collector runs to
make sure everything is cleaned up if you’ve been messy with circular
references and such.)

=item GID

Group ID—in Unix, X<GID (Group ID)>X<Group ID (GID)>the numeric group ID
that the B<operating system> uses to identify you and members of your
B<group>.

=item glob

Strictly, the X<glob (* character)>shell’s C<*> character, which will match
a “glob” of characters when you’re trying to generate a list of filenames.
Loosely, the act of using globs and similar symbols to do pattern matching.
See also B<fileglob> and B<typeglob>.

=item global

Something X<global (term)>you can see from anywhere, usually used of
B<variables> and B<subroutines> that are visible everywhere in your
program.  In Perl, only certain special variables are truly global—most
variables (and all subroutines) exist only in the current B<package>.
Global variables can be declared with C<our>. See “Global Declarations” in
Camel chapter 4, “Statements and Declarations”.

=item global destruction

The B<garbage X<global destruction>collection> of globals (and the running
of any associated object destructors) that takes place when a Perl
B<interpreter> is being shut down. Global destruction should not be
confused with the Apocalypse, except perhaps when it should.

=item glue language

A language X<glue language>such as Perl that is good at hooking things
together that weren’t intended to be hooked together.

=item granularity

The size of the X<granularity>pieces you’re dealing with, mentally
speaking.

=item grapheme

A graphene is X<graphemes, defined>an allotrope of carbon arranged in a
hexagonal crystal lattice one atom thick. A B<grapheme>, or more fully, a
I<grapheme cluster string> is a single user-visible B<character>, which may
in turn be several characters (B<codepoints>) long. For example, a carriage
return plus a line feed is a single grapheme but two characters, while a
“ȫ” is a single grapheme but one, two, or even three characters, depending
on B<normalization>.

=item greedy

A B<subpattern> X<greedy subpatterns>X<subpatterns, greedy>whose
B<quantifier> wants to match as many things as possible.

=item grep

Originally X<grep function>from the old Unix editor command for “Globally
search for a Regular Expression and Print it”, now used in the general
sense of any kind of search, especially text searches. Perl has a built-in
C<grep> function that searches a list for elements matching any given
criterion, whereas the B<grep>(1) program searches for lines matching a
B<regular expression> in one or more files.

=item group

A set of users X<groups, defined>of which you are a member. In some
operating systems (like Unix), you can give certain file access permissions
to other members of your group.

=item GV

An internal “glob value” X<GV (glob value)>X<glob value (GV)>typedef,
holding a B<typeglob>. The C<GV> type is a subclass of B<SV>.

=back

=head2 H

=over 4

=item hacker

Someone X<hackers>who is brilliantly persistent in solving technical
problems, whether these involve golfing, fighting orcs, or programming.
Hacker is a neutral term, morally speaking. Good hackers are not to be
confused with evil B<crackers> or clueless B<script kiddies>. If you
confuse them, we will presume that you are either evil or clueless.

=item handler

A B<subroutine> X<handlers, defined>or B<method> that Perl calls when your
program needs to respond to some internal event, such as a B<signal>, or an
encounter with an operator subject to B<operator overloading>. See also
B<callback>.

=item hard reference

A B<scalar> B<value> X<hard references, about>X<references, hard>containing
the actual address of a B<referent>, such that the referent’s B<reference>
count accounts for it. (Some hard references are held internally, such as
the implicit reference from one of a B<typeglob>’s variable slots to its
corresponding referent.) A hard reference is different from a B<symbolic
reference>.

=item hash

An unordered X<hashes, about>association of B<key>/B<value> X<key/value
pairs, about>pairs, stored such that you can easily use a string B<key> to
look up its associated data B<value>. This glossary is like a hash, where
the word to be defined is the key and the definition is the value. A hash
is also sometimes septisyllabically called an “associative array”, which is
a pretty good reason for simply calling it a “hash” instead.

=item hash table

A data X<hash tables>structure used internally by Perl for implementing
associative arrays (hashes) efficiently. See also B<bucket>.

=item header file

A file X<header files>X<files, header>containing certain required
definitions that you must include “ahead” of the rest of your program to do
certain obscure operations. A C header file has a I<.h> extension. Perl
doesn’t really have header files, though historically Perl has sometimes
used translated I<.h> files with a I<.ph> extension. See C<require> in
Camel chapter 27, “Functions”. (Header files have been superseded by the
B<module> mechanism.)

=item here document

So X<here documents>called because of a similar construct in B<shells> that
pretends that the B<lines> following the B<command> are a separate B<file>
to be fed to the command, up to some terminating string. In Perl, however,
it’s just a fancy form of quoting.

=item hexadecimal

A X<hexadecimals>number in base 16, “hex” for short. The digits for 10
through 15 are customarily represented by the letters C<a> through C<f>.
Hexadecimal constants in Perl start with C<0x>. See also the C<hex>
function in Camel chapter 27, “Functions”.

=item home directory

The X<home directory>X<directories, home>directory you are put into when
you log in. On a Unix system, the name is often placed into C<$ENV{HOME}>
or C<$ENV{LOGDIR}> by I<login>, but you can also find it with
C<(get>C<pwuid($E<lt>))[7]>. (Some platforms do not have a concept of a
home directory.)

=item host

The computer X<host computers>on which a program or other data resides.

=item hubris

Excessive pride, X<hubris quality>the sort of thing for which Zeus zaps
you.  Also the quality that makes you write (and maintain) programs that
other people won’t want to say bad things about. Hence, the third great
virtue of a programmer. See also B<laziness> and B<impatience>.

=item HV

Short for a “hash value” X<HV (hash value)>X<hash value (HV)>typedef, which
holds Perl’s internal representation of a hash. The C<HV> type is a
subclass of B<SV>.

=back

=head2 I

=over 4

=item identifier

A legally X<identifiers, defined>formed name for most anything in which a
computer program might be interested. Many languages (including Perl) allow
identifiers to start with an alphabetic character, and then contain
alphabetics and digits. Perl also allows connector punctuation like the
underscore character wherever it allows alphabetics. (Perl also has more
complicated names, like B<qualified> names.)

=item impatience

The anger X<impatience quality>you feel when the computer is being lazy.
This makes you write programs that don’t just react to your needs, but
actually anticipate them. Or at least that pretend to. Hence, the second
great virtue of a programmer. See also B<laziness> and B<hubris>.

=item implementation

How a X<implementation (term)>piece of code actually goes about doing its
job. Users of the code should not count on implementation details staying
the same unless they are part of the published B<interface>.

=item import

To gain X<import (term)>access to symbols that are exported from another
module. See C<use> in Camel chapter 27, “Functions”.

=item increment

To increase the X<incrementing values>X<values, incrementing>value of
something by 1 (or by some other number, if so specified).

=item indexing

In olden days, X<indexing (term)>the act of looking up a B<key> in an
actual index (such as a phone book). But now it's merely the act of using
any kind of key or position to find the corresponding B<value>, even if no
index is involved. Things have degenerated to the point that Perl’s
C<index> function merely locates the position (index) of one string in
another.

=item indirect filehandle

An B<expression> X<indirect filehandles>X<filehandles, indirect>that
evaluates to something that can be used as a B<filehandle>: a B<string>
(filehandle name), a B<typeglob>, a typeglob B<reference>, or a low-level
B<IO> object.

=item indirection

If something in a X<indirection (term)>program isn’t the value you’re
looking for but indicates where the value is, that’s indirection. This can
be done with either B<symbolic references> or B<hard>.

=item indirect object

In English grammar, X<indirect objects, defined>X<objects, indirect>a short
noun phrase between a verb and its direct object indicating the beneficiary
or recipient of the action. In Perl, C<print STDOUT "$foo\n";> can be
understood as “verb indirect-object object”, where C<STDOUT> is the
recipient of the C<print> action, and C<"$foo"> is the object being
printed.  Similarly, when invoking a B<method>, you might place the
invocant in the dative slot between the method and its arguments:

    $gollum = new Pathetic::Creature "Sméagol";
    give $gollum "Fisssssh!";
    give $gollum "Precious!";

=item indirect object slot

The syntactic X<indirect object slot>position falling between a method call
and its arguments when using the indirect object invocation syntax. (The
slot is distinguished by the absence of a comma between it and the next
argument.) C<STDERR> is in the indirect object slot here:

    print STDERR "Awake! Awake! Fear, Fire, Foes! Awake!\n";

=item infix

An B<operator> that X<infix operators>comes in between its B<operands>,
such as multiplication in C<24 * 7>.

=item inheritance

What you get from your X<inheritance, defined>ancestors, genetically or
otherwise. If you happen to be a B<class>, your ancestors are called B<base
classes> and your descendants are called B<derived classes>. See B<single
inheritance> and B<multiple inheritance>.

=item instance

Short for “an instance of a class”, X<instances (term)>meaning an B<object>
of that B<class>.

=item instance data

SeeX<instance data> B<instance variable>.

=item instance method

A B<method> of X<instance methods>X<methods, instance>an B<object>, as
opposed to a B<class method>.

A B<method> whose B<invocant> is an B<object>, not a B<package> name. Every
object of a class shares all the methods of that class, so an instance
method applies to all instances of the class, rather than applying to a
particular instance. Also see B<class method>.

=item instance variable

An B<attribute> of an B<object>; X<instance variables, defined>X<variables,
instance>data stored with the particular object rather than with the class
as a whole.

=item integer

A number X<integers (term)>with no fractional (decimal) part. A counting
number, like 1, 2, 3, and so on, but including 0 and the negatives.

=item interface

The services X<interfaces (term)>a piece of code promises to provide
forever, in contrast to its B<implementation>, which it should feel free to
change whenever it likes.

=item interpolation

The insertion of X<interpolation, defined>a scalar or list value somewhere
in the middle of another value, such that it appears to have been there all
along. In Perl, variable interpolation happens in double-quoted strings and
patterns, and list interpolation occurs when constructing the list of
values to pass to a list operator or other such construct that takes a
I<C<LIST>>.

=item interpreter

Strictly speaking, X<interpreters, defined>a program that reads a second
program and does what the second program says directly without turning the
program into a different form first, which is what B<compilers> do. Perl is
not an interpreter by this definition, because it contains a kind of
compiler that takes a program and turns it into a more executable form
(B<syntax trees>) within the I<perl> process itself, which the Perl
B<runtime> system then interprets.

=item invocant

The agent on X<invocants, defined>whose behalf a B<method> is invoked. In a
B<class> method, the invocant is a package name. In an B<instance> method,
the invocant is an object reference.

=item invocation

The act of X<invocation, method>calling up a deity, daemon, program,
method, subroutine, or function to get it to do what you think it’s
supposed to do.  We usually “call” subroutines but “invoke” methods, since
it sounds cooler.

=item I/O

Input from, or X<I/O (Input/Output), defined>X<Input/Output (I/O),
defined>output to, a B<file> or B<device>.

=item IO

An internal I/O object. Can also mean B<indirect object>.

=item I/O layer

One of the X<I/O layer>filters between the data and what you get as input
or what you end up with as output.

=item IPA

India Pale Ale. Also the X<International Phonetic Alphabet (IPA)>X<IPA
(International Phonetic Alphabet)>International Phonetic Alphabet, the
standard alphabet used for phonetic notation worldwide. Draws heavily on
Unicode, including many combining characters.

=item IP

Internet ProtocolX<Internet Protocol (IP)>X<IP (Internet Protocol)>, or
X<IP (Intellectual Property)>X<Intellectual Property (IP)>Intellectual
Property.

=item IPC

Interprocess X<Interprocess Communication>X<IPC (Interprocess
Communication), about>X<communication>Communication.

=item is-a

A rX<is–a relationship>elationship between two B<objects> in which one
object is considered to be a more specific version of the other, generic
object: “A camel is a mammal.” Since the generic object really only exists
in a Platonic sense, we usually add a little abstraction to the notion of
objects and think of the relationship as being between a generic B<base
class> and a specific B<derived class>. Oddly enough, Platonic classes
don’t always have Platonic relationships—see B<inheritance>.

=item iteration

Doing X<iteration>something repeatedly.

=item iterator

A special X<iterators>programming gizmo that keeps track of where you are
in something that you’re trying to iterate over. The C<foreach> loop in
Perl contains an iterator; so does a hash, allowing you to C<each> through
it.

=item IV

The integer X<IV (Integer Value)>X<Integer Value (IV)>four, not to be
confused with six, Tom’s favorite editor. IV also means an internal Integer
Value of the type a B<scalar> can hold, not to be confused with an B<NV>.

=back

=head2 J

=over 4

=item JAPH

“Just Another Perl Hacker”, a X<JAPH acronym>clever but cryptic bit of Perl
code that, when executed, evaluates to that string. Often used to
illustrate a particular Perl feature, and something of an ongoing
Obfuscated Perl Contest seen in USENET signatures.

=back

=head2 K

=over 4

=item key

The X<keys, defined>string index to a B<hash>, used to look up the B<value>
associated with that key.

=item keyword

See B<reserved words>.

=back

=head2 L

=over 4

=item label

A X<labels, defined>name you give to a B<statement> so that you can talk
about that statement elsewhere in the program.

=item laziness

The X<laziness quality>quality that makes you go to great effort to reduce
overall energy expenditure. It makes you write labor-saving programs that
other people will find useful, and then document what you wrote so you
don’t have to answer so many questions about it. Hence, the first great
virtue of a programmer. Also hence, this book. See also B<impatience> and
B<hubris>.

=item leftmost longest

The X<leftmost longest preference>X<regular expressions, leftmost longest
preference>preference of the B<regular expression> engine to match the
leftmost occurrence of a B<pattern>, then given a position at which a match
will occur, the preference for the longest match (presuming the use of a
B<greedy> quantifier). See Camel chapter 5, “Pattern Matching” for I<much>
more on this subject.

=item left shift

A B<bit shift> that X<left shift (E<lt>E<lt>) bit operator>X<bit–shift
operators, left shift>X<E<lt>E<lt> (left shift) bit operator>multiplies the
number by some power of 2.

=item lexeme

Fancy X<lexeme (token)>term for a B<token>.

=item lexer

Fancy X<lexer (tokener)>term for a B<tokener>.

=item lexical analysis

Fancy X<lexical analysis>term for B<tokenizing>.

=item lexical scoping

Looking X<lexical scopes, defined>X<scopes>at your I<Oxford English
Dictionary> through a microscope. (Also known as B<static scoping>, because
dictionaries don’t change very fast.) Similarly, looking at variables
stored in a private dictionary (namespace) for each scope, which are
visible only from their point of declaration down to the end of theX<static
scopes>X<scopes, static> lexical scope in which they are declared. —Syn.
B<static scoping>. —Ant. B<dynamic scoping>.

=item lexical variable

A B<variable> X<lexical variables, about>X<variables, lexical>subject to
B<lexical scoping>, declared by C<my>. Often just called a “lexical”. (The
C<our> declaration declares a lexically scoped name for a global variable,
which is not itself a lexical variable.)

=item library

Generally, a X<libraries, defined>collection of procedures. In ancient
days, referred to a collection of subroutines in a I<.pl> file. In modern
times, refers more often to the entire collection of Perl B<modules> on
your system.

=item LIFO

Last In, First OutX<Last In, First Out (LIFO)>X<LIFO (Last In, First
Out)>X<stacks, defined>. See also B<FIFO>. A LIFO is usually called a
B<stack>.

=item line

In Unix, a X<line (term)>sequence of zero or more nonnewline characters
terminated with a B<newline> character. On non-Unix machines, this is
emulated by the C library even if the underlying B<operating system> has
different ideas.

=item linebreak

A B<grapheme> X<linebreaks>consisting of either a carriage return followed
by a line feed or any character with the Unicode Vertical Space B<character
property>.

=item line buffering

Used by X<line buffering>X<buffering, line>a B<standard I/O> output stream that
flushes its B<buffer> after every B<newline>. Many standard I/O libraries
automatically set up line buffering on output that is going to the terminal.

=item line number

The number X<line number>of lines read previous to this one, plus 1. Perl
keeps a separate line number for each source or input file it opens. The
current source file’s line number is represented by C<__LINE__>. The
current input line number (for the file that was most recently read via
C<E<lt>FHE<gt>>) is represented by the C<$.> (C<$INPUT_LINE_NUMBER>)
variable. Many error messages report both values, if available.

=item link

Used as a X<links, defined>noun, a name in a B<directory> that represents a
B<file>. A given file can have multiple links to it. It’s like having the
same phone number listed in the phone directory under different names. As a
verb, to resolve a partially B<compiled> file’s unresolved symbols into a
(nearly) executable image. Linking can generally be static or dynamic,
which has nothing to do with static or dynamic scoping.

=item LIST

A syntactic X<LIST construct>X<constructs, LIST>construct representing a
comma- separated list of expressions, evaluated to produce a B<list value>.
Each B<expression> in a I<C<LIST>> is evaluated in B<list context> and
interpolated into the list value.

=item list

An ordered X<lists, defined>set of scalar values.

=item list context

The situation X<list context>X<context, list>in which an B<expression> is
expected by its surroundings (the code calling it) to return a list of
values rather than a single value. Functions that want a I<C<LIST>> of
arguments tell those arguments that they should produce a list value. See
also B<context>.

=item list operator

An B<operator> that X<list operators, about>does something with a list of
values, such as C<join> or C<grep>. Usually used for named built-in
operators (such as C<print>, C<unlink>, and C<system>) that do not require
parentheses around their B<argument> list.

=item list value

An unnamed X<list values, about>X<values, list>list of temporary scalar
values that may be passed around within a program from any list-generating
function to any function or construct that provides a B<list context>.

=item literal

A token X<literals, defined>in a programming language, such as a number or
B<string>, that gives you an actual B<value> instead of merely representing
possible values as a B<variable> does.

=item little-endian

From Swift: X<little–endian, defined>X<endianness, little–endian>someone
who eats eggs little end first. Also used of computers that store the least
significant B<byte> of a word at a lower byte address than the most
significant byte. Often considered superior to big-endian machines. See
also B<big-endian>.

=item local

Not meaning X<local operator, about>the same thing everywhere. A global
variable in Perl can be localized inside a B<dynamic scope> via the
C<local> operator.

=item logical operator

Symbols X<logical operators, about>representing the concepts “and”, “or”,
“xor”, and “not”.

=item lookahead

An B<assertion> that X<lookahead assertions>X<assertions (in regexes),
lookahead>peeks at the string to the right of the current match location.

=item lookbehind

An B<assertion> X<lookbehind assertions>X<assertions (in regexes),
lookbehind>that peeks at the string to the left of the current match
location.

=item loop

A construct X<loop constructs and statements, about>X<constructs, loop>that
performs something repeatedly, like a roller coaster.

=item loop control statement

Any statement X<statements, loop control>within the body of a loop that can
make a loop prematurely stop looping or skip an B<iteration>. Generally,
you shouldn’t try this on roller coasters.

=item loop label

A kind X<loop labels>X<labels, loop>of key or name attached to a loop (or
roller coaster) so that loop control statements can talk about which loop
they want to control.

=item lowercase

In Unicode, X<lowercase characters>X<characters, lowercase>not just
characters with the General Category of Lowercase Letter, but any character
with the Lowercase property, including Modifier Letters, Letter Numbers,
some Other Symbols, and one Combining Mark.

=item lvaluable

Able to X<lvaluable function>X<functions, lvaluable>serve as an B<lvalue>.

=item lvalue

Term used by X<lvalue (term)>X<values, lvalue>language lawyers for a
storage location you can assign a new B<value> to, such as a B<variable> or
an element of an B<array>. The “l” is short for “left”, as in the left side
of an assignment, a typical place for lvalues. An B<lvaluable> function or
expression is one to which a value may be assigned, as in C<pos($x) = 10>.

=item lvalue modifier

An X<lvalue modifier>X<modifiers, lvalue>adjectival pseudofunction that
warps the meaning of an B<lvalue> in some declarative fashion. Currently
there are three lvalue modifiers: C<my>, C<our>, and C<local>.

=back

=head2 M

=over 4

=item magic

Technically X<magic (term)>speaking, any extra semantics attached to a
variable such as C<$!>, C<$0>, C<%ENV>, or C<%SIG>, or to any tied
variable.  Magical things happen when you diddle those variables.

=item magical increment

An B<increment> X<magical increment operator>operator that knows how to
bump up ASCII alphabetics as well as numbers.

=item magical variables

Special variables X<magical variables>X<variables, magical>that have side
effects when you access them or assign to them. For example, in Perl,
changing elements of the C<%ENV> array also changes the corresponding
environment variables that subprocesses will use. Reading the C<$!>
variable gives you the current system error number or message.

=item Makefile

A file that X<Makefile>controls the compilation of a program. Perl programs
don’t usually need a B<Makefile> because the Perl compiler has plenty of
self-control.

=item man

The Unix X<man program (Unix)>program that displays online documentation
(manual pages) for you.

=item manpage

A “page” from the X<manpages, defined>manuals, typically accessed via the
I<man>(1) command. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of
BUGS, and so on, and is typically longer than a page. There are manpages
documenting B<commands>, B<syscalls>, B<library> B<functions>, B<devices>,
B<protocols>, B<files>, and such. In this book, we call any piece of
standard Perl documentation (like L<perlop> or L<perldelta>) a manpage, no
matter what format it’s installed in on your system.

=item matching

SeeX<matching> B<pattern matching>.

=item member data

SeeX<member data> B<instance variable>.

=item memory

This X<memory, defined>always means your main memory, not your disk.
Clouding the issue is the fact that your machine may implement
B<virtual> memory; that is, it will pretend that it has more memory than
it really does, and it’ll use disk space to hold inactive bits. This can
make it seem like you have a little more memory than you really do, but
it’s not a substitute for real memory. The best thing that can be said
about virtual memory is that it lets your performance degrade gradually
rather than suddenly when you run out of real memory. But your program
can die when you run out of virtual memory, too—if you haven’t thrashed
your disk to death first.

=item metacharacter

A B<character> that X<metacharacters, about>X<characters, regex
metacharacters>is I<not> supposed to be treated normally. Which characters
are to be treated specially as metacharacters varies greatly from context to
context. Your B<shell> will have certain metacharacters, double-quoted Perl
B<strings> have other metacharacters,X<regular expressions, metacharacters and>
and B<regular expression> patterns have all the double-quote metacharacters plus
some extra ones of their own.

=item metasymbol

Something we’d call X<metasymbols, about>X<escape sequences>a
B<metacharacter> except that it’s a sequence of more than one character.
Generally, the first character in the sequence must be a true metacharacter
to get the other characters in the metasymbol to misbehave along with it.

=item method

A kind of X<methods, defined>action that an B<object> can take if you tell
it to. See Camel chapter 12, “Objects”.

=item method resolution order

The path X<method resolution order (mro)>X<mro (method resolution
order)>Perl takes through C<@INC>. By default, this is a double depth first
search, once looking for defined methods and once for C<AUTOLOAD>. However,
Perl lets you configure this with C<mro>.

=item minicpan

A CPAN X<minicpan, defined>X<CPAN (Comprehensive Perl Archive Network),
minicpan and>mirror that includes just the latest versions for each
distribution, probably created with C<CPAN::Mini>X<CPAN::Mini module>. See
Camel chapter 19, “CPAN”.

=item minimalism

The belief X<minimalism>that “small is beautiful”. Paradoxically, if you
say something in a small language, it turns out big, and if you say it in a
big language, it turns out small. Go figure.

=item mode

In the X<mode>context of the I<stat>(2) syscall, refers to the field
holding the B<permission bits> and the type of the B<file>.

=item modifier

SeeX<modifiers, defined> B<statement modifier>, B<regular expression>, and
B<lvalue>, not necessarily in that order.

=item module

A B<file> that X<modules, defined>defines a B<package> of (almost) the same
name, which can either B<export> symbols or function as an B<object> class.
(A module’s main I<.pm> file may also load in other files in support of the
module.) See the C<use> built-in.

=item modulus

An integer X<modulus (%) operator>X<% (modulus) operator>divisor when
you’re interested in the remainder instead of the quotient.

=item mojibake

When you X<mojibake>speak one language and the computer thinks you’re
speaking another. You’ll see odd translations when you send UTF‑8, for
instance, but the computer thinks you sent Latin-1, showing all sorts of
weird characters instead. The term is written 「文字化け」in Japanese and
means “character rot”, an apt description. Pronounced [C<modʑibake>] in
standard B<IPA> phonetics, or approximately “moh-jee-bah-keh”.

=item monger

Short for X<mongers, Perl>X<Perl mongers>one member of B<Perl mongers>, a
purveyor of Perl.

=item mortal

A temporary X<mortal value>X<values, mortal>value scheduled to die when the
current statement finishes.

=item mro

See B<method resolution order>.

=item multidimensional array

An array X<multidimensional arrays>X<arrays, multidimensional>with multiple
subscripts for finding a single element. Perl implements these using
B<references>—see Camel chapter 9, “Data Structures”.

=item multiple inheritance

The features X<multiple inheritance>X<inheritance, multiple>you got from
your mother and father, mixed together unpredictably. (See also
B<inheritance> and B<single inheritance>.) In computer languages (including
Perl), it is the notion that a given class may have multiple direct
ancestors or B<base classes>.

=back

=head2 N

=over 4

=item named pipe

A B<pipe> X<named pipes>X<pipes, names>with a name embedded in the
B<filesystem> so that it can be accessed by two unrelated B<processes>.

=item namespace

A domain of X<namespaces, about>names. You needn’t worry about whether the
names in one such domain have been used in another. See B<package>.

=item NaN

Not a number. X<NaN (not a number)>X<not a number (NaN)>The value Perl uses
for certain invalid or inexpressible floating-point operations.

=item network address

The most X<network address>important attribute of a socket, like your
telephone’s telephone number. Typically an IP address. See also B<port>.

=item newline

A single X<newline character>X<characters, newline>character that
represents the end of a line, with the ASCII value of 012 octal under Unix
(but 015 on a Mac), and represented by C<\n> in Perl strings. For Windows
machines writing text files, and for certain physical devices like
terminals, the single newline gets automatically translated by your C
library into a line feed and a carriage return, but normally, no
translation is done.

=item NFS

Network File System, X<NFS (Network File System)>X<Network File System
(NFS)>which allows you to mount a remote filesystem as if it were local.

=item normalization

Converting a X<normalization>text string into an alternate but equivalent
B<canonical> (or compatible) representation that can then be compared for
equivalence. Unicode recognizes four different normalization forms: NFD,
NFC, NFKD, and NFKC.

=item null character

A character X<null character>X<characters, null>with the numeric value of
zero. It’s used by C to terminate strings, but Perl allows strings to
contain a null.

=item null list

A B<list value> with X<null lists>X<lists, null>zero elements, represented
in Perl by C<()>.

=item null string

A B<string> X<null strings>X<strings, null>containing no characters, not to
be confused with a string containing a B<null character>, which has a
positive length and is B<true>.

=item numeric context

The situation X<numeric context>X<context, numeric>in which an expression
is expected by its surroundings (the code calling it) to return a number.
See also B<context> and B<string context>.

=item numification

(Sometimes spelled I<nummification> and I<nummify>.) X<numification>Perl lingo
for implicit conversion into a number; the related verb is I<numify>.
I<Numification> is intended to rhyme with I<mummification>, and I<numify> with
I<mummify>. It is unrelated to English I<numen>, I<numina>, I<numinous>. We
originally forgot the extra I<m> a long time ago, and some people got used to
our funny spelling, and so just as with C<HTTP_REFERER>’s own missing letter,
our weird spelling has stuck around.

=item NV

Short for Nevada, X<Numeric Value (NV)>X<NV (Numeric Value)>no part of
which will ever be confused with civilization. NV also means an internal
floating- point Numeric Value of the type a B<scalar> can hold, not to be
confused with an B<IV>.

=item nybble

Half a B<byte>, X<nybble>equivalent to one B<hexadecimal> digit, and worth
four B<bits>.

=back

=head2 O

=over 4

=item object

An B<instance> X<objects, defined>of a B<class>. Something that “knows”
what user-defined type (class) it is, and what it can do because of what
class it is. Your program can request an object to do things, but the
object gets to decide whether it wants to do them or not. Some objects are
more accommodating than others.

=item octal

A number X<octals>in base 8. Only the digits 0 through 7 are allowed. Octal
constants in Perl start with 0, as in 013. See also the C<oct> function.

=item offset

How many X<offsets in strings>X<strings, offsets in>things you have to skip
over when moving from the beginning of a string or array to a specific
position within it. Thus, the minimum offset is zero, not one, because you
don’t skip anything to get to the first item.

=item one-liner

An entire X<one–liner programs>computer program crammed into one line of
text.

=item open source software

Programs X<open source software>for which the source code is freely
available and freely redistributable, with no commercial strings attached.
For a more detailed definition, see L<http://www.opensource.org/osd.html>.

=item operand

An B<expression> X<operands (term)>that yields a B<value> that an
B<operator> operates on. See also B<precedence>.

=item operating system

A special X<operating systems, defined>program that runs on the bare
machine and hides the gory details of managing B<processes> and B<devices>.
Usually used in a looser sense to indicate a particular culture of
programming. The loose sense can be used at varying levels of specificity.
At one extreme, you might say that all versions of Unix and Unix-lookalikes
are the same operating system (upsetting many people, especially lawyers
and other advocates). At the other extreme, you could say this particular
version of this particular vendor’s operating system is different from any
other version of this or any other vendor’s operating system. Perl is much
more portable across operating systems than many other languages. See also
B<architecture> and B<platform>.

=item operator

A gizmo X<operators, about>that transforms some number of input values to
some number of output values, often built into a language with a special
syntax or symbol. A given operator may have specific expectations about
what B<types> of data you give as its arguments (B<operands>) and what type
of data you want back from it.

=item operator overloading

A kind X<operator overloading, about>X<overloading, operator>of
B<overloading> that you can do on built-in B<operators> to make them work
on B<objects> as if the objects were ordinary scalar values, but with the
actual semantics supplied by the object class. This is set up with the
overload B<pragma>—see Camel chapter 13, “Overloading”.

=item options

See X<options>either B<switches> or B<regular expression modifiers>.

=item ordinal

An X<ordinals (term)>abstract character’s integer value. Same thing as
B<codepoint>.

=item overloading

Giving X<overloading, defined>additional meanings to a symbol or construct.
Actually, all languages do overloading to one extent or another, since
people are good at figuring out things from B<context>.

=item overriding

Hiding or X<overriding, defined>invalidating some other definition of the
same name. (Not to be confused with B<overloading>, which adds definitions
that must be disambiguated some other way.) To confuse the issue further,
we use the word with two overloaded definitions: to describe how you can
define your own B<subroutine> to hide a built-in B<function> of the same
name (see the section “Overriding Built-in Functions” in Camel chapter 11,
“Modules”), and to describe how you can define a replacement B<method> in a
B<derived class> to hide a B<base class>’s method of the same name (see
Camel chapter 12, “Objects”).

=item owner

The one X<ownership, file>X<files, ownership of>user (apart from the
superuser) who has absolute control over a B<file>. A file may also have a
B<group> of users who may exercise joint ownership if the real owner
permits it. See B<permission bits>.

=back

=head2 P

=over 4

=item package

A B<namespace> for X<packages, defined>global B<variables>, B<subroutines>,
and the like, such that they can be kept separate from like-named
B<symbols> in other namespaces. In a sense, only the package is global,
since the symbols in the package’s symbol table are only accessible from
code B<compiled> outside the package by naming the package. But in another
sense, all package symbols are also globals—they’re just well-organized
globals.

=item pad

Short X<pads (scratchpads)>for B<scratchpad>.

=item parameter

SeeX<parameters> B<argument>.

=item parent class

SeeX<parent classes>X<classes, parent> B<base class>.

=item parse tree

SeeX<parse tree> B<syntax tree>.

=item parsing

The X<parsing, about>subtle but sometimes brutal art of attempting to turn
your possibly malformed program into a valid B<syntax tree>.

=item patch

To X<patches>fix by applying one, as it were. In the realm of hackerdom, a
listing of the differences between two versions of a program as might be
applied by the B<patch>(1) program when you want to fix a bug or upgrade
your old version.

=item PATH

The X<PATH environment variable>X<variables, environment>list of
B<directories> the system searches to find a program you want to
B<execute>.  The list is stored as one of your B<environment variables>,
accessible in Perl as C<$ENV{PATH}>.

=item pathname

A X<pathname>fully qualified filename such as I</usr/bin/perl>. Sometimes
confused with C<PATH>.

=item pattern

A X<patterns, defined>template used in B<pattern matching>.

=item pattern matching

Taking a X<pattern matching, about>pattern, usually a B<regular
expression>, and trying the pattern various ways on a string to see whether
there’s any way to make it fit. Often used to pick interesting tidbits out
of a file.

=item PAUSE

The X<Perl Authors Upload SErver (PAUSE)>X<PAUSE (Perl Authors Upload
SErver)>Perl Authors Upload SErver (L<http://pause.perl.org>), the gateway
for B<modules> on their way to B<CPAN>.

=item Perl mongers

A X<Perl mongers>X<mongers, Perl>Perl user group, taking the form of its
name from the New York Perl mongers, the first Perl user group. Find one
near you at L<http://www.pm.org>.

=item permission bits

Bits X<permission bits>X<bits, permission>that the B<owner> of a file sets
or unsets to allow or disallow access to other people. These flag bits are
part of the B<mode> word returned by the C<stat> built-in when you ask
about a file. On Unix systems, you can check the I<ls>(1) manpage for more
information.

=item Pern

What you get X<Pern (term)>when you do C<Perl++> twice. Doing it only once
will curl your hair. You have to increment it eight times to shampoo your
hair. Lather, rinse, iterate.

=item pipe

A X<pipes, defined>direct B<connection> that carries the output of one
B<process> to the input of another without an intermediate temporary file.
Once the pipe is set up, the two processes in question can read and write
as if they were talking to a normal file, with some caveats.

=item pipeline

A X<pipeline>series of B<processes> all in a row, linked by B<pipes>, where
each passes its output stream to the next.

=item platform

The X<platforms, defined>entire hardware and software context in which a
program runs. A program written in a platform-dependent language might
break if you change any of the following: machine, operating system,
libraries, compiler, or system configuration. The I<perl> interpreter has
to be B<compiled> differently for each platform because it is implemented
in C, but programs written in the Perl language are largely platform
independent.

=item pod

The X<pod (plain old documentation), about>X<plain old documentation>markup
used to embed documentation into your Perl code. Pod stands for “Plain old
documentation”. See Camel chapter 23, “Plain Old Documentation”.

=item pod command

A X<pod commands>X<commands, pod>sequence, such as C<=head1>, that denotes
the start of a B<pod> section.

=item pointer

A B<variable> X<pointers>in a language like C that contains the exact
memory location of some other item. Perl handles pointers internally so you
don’t have to worry about them. Instead, you just use symbolic pointers in
the form of B<keys> and B<variable> names, or B<hard references>, which
aren’t pointers (but act like pointers and do in fact contain pointers).

=item polymorphism

The notion X<polymorphism>that you can tell an B<object> to do something
generic, and the object will interpret the command in different ways
depending on its type. [E<lt> Greek πολυ- + μορϕή, many forms.]

=item port

The X<ports (term)>part of the address of a TCP or UDP socket that directs
packets to the correct process after finding the right machine, something
like the phone extension you give when you reach the company operator. Also
the result of converting code to run on a different platform than
originally intended, or the verb denoting this conversion.

=item portable

Once X<portability, about>upon a time, C code compilable under both BSD and
SysV. In general, code that can be easily converted to run on another
B<platform>, where “easily” can be defined however you like, and usually
is.  Anything may be considered portable if you try hard enough, such as a
mobile home or London Bridge.

=item porter

Someone X<porters>who “carries” software from one B<platform> to another.
Porting programs written in platform-dependent languages such as C can be
difficult work, but porting programs like Perl is very much worth the
agony.

=item possessive

Said of X<possessive (term)>quantifiers and groups in patterns that refuse
to give up anything once they’ve gotten their mitts on it. Catchier and
easier to say than the even more formal I<nonbacktrackable>.

=item POSIX

The X<Portable Operating System Interface (POSIX), about>X<POSIX (Portable
Operating System Interface), about>Portable Operating System Interface
specification.

=item postfix

An B<operator> X<postfix operator>that follows its B<operand>, as in
C<$x++>.

=item pp

An X<pp (push–pop) code>X<push–pop (pp) code>internal shorthand for a
“push- pop” code; that is, C code implementing Perl’s stack machine.

=item pragma

A X<pragmas, about>X<modules>standard module whose practical hints and
suggestions are received (and possibly ignored) at compile time. Pragmas
are named in all lowercase.

=item precedence

The X<precedence rules, about>X<operators, precedence rules>rules of
conduct that, in the absence of other guidance, determine what should
happen first.  For example, in the absence of parentheses, you always do
multiplication before addition.

=item prefix

An B<operator> X<prefix operators>that precedes its B<operand>, as in
C<++$x>.

=item preprocessing

What X<preprocessing>some helper B<process> did to transform the incoming
data into a form more suitable for the current process. Often done with an
incoming B<pipe>. See also B<C preprocessor>.

=item primary maintainer

The X<primary maintainer>author that PAUSE allows to assign B<co-maintainer>
permissions to a B<namespace>. A primary maintainer can give up this
distinction by assigning it to another PAUSE author. See Camel chapter 19,
“CPAN”.

=item procedure

AX<procedures, defined> B<subroutine>.

=item process

An X<processes, defined>instance of a running program. Under multitasking
systems like Unix, two or more separate processes could be running the same
program independently at the same time—in fact, the C<fork> function is
designed to bring about this happy state of affairs. Under other operating
systems, processes are sometimes called “threads”, “tasks”, or “jobs”,
often with slight nuances in meaning.

=item program

See B<script>.

=item program generator

A system X<program generators>that algorithmically writes code for you in a
high-level language. See also B<code generator>.

=item progressive matching

B<Pattern matching> X<progressive matching>X<pattern matching, progressive
matching> matching>that picks up where it left off before.

=item property

See X<property>either B<instance variable> or B<character property>.

=item protocol

In X<protocols (term)>networking, an agreed-upon way of sending messages
back and forth so that neither correspondent will get too confused.

=item prototype

An X<prototypes, about>optional part of a B<subroutine> declaration telling
the Perl compiler how many and what flavor of arguments may be passed as
B<actual arguments>, so you can write subroutine calls that parse much like
built-in functions. (Or don’t parse, as the case may be.)

=item pseudofunction

A X<pseudofunctions>X<constructs, pseudofunctions>X<functions,
pseudofunctions>construct that sometimes looks like a function but really
isn’t. Usually reserved for B<lvalue> modifiers like C<my>, for B<context>
modifiers like C<scalar>, and for the pick-your-own-quotes constructs,
C<q//>, C<qq//>, C<qx//>, C<qw//>, C<qr//>, C<m//>, C<s///>, C<y///>, and
C<tr///>.

=item pseudohash

Formerly, a reference X<pseudohashes>X<hashes, pseudohashes>to an array
whose initial element happens to hold a reference to a hash. You used to be
able to treat a pseudohash reference as either an array reference or a hash
reference. Pseudohashes are no longer supported.

=item pseudoliteral

An B<operator> X<pseudoliterals>XC<that looks something like a B<literal>,
such as the output-grabbing operator, <literal
moreinfo="none">`>I<C<command>>C<`>.

=item public domain

Something X<public domain>not owned by anybody. Perl is copyrighted and is
thus I<not> in the public domain—it’s just B<freely available> and B<freely
redistributable>.

=item pumpkin

A X<pumpkin (term)>notional “baton” handed around the Perl community
indicating who is the lead integrator in some arena of development.

=item pumpking

A B<X<pumpking>pumpkin> holder, the person in charge of pumping the pump,
or at least priming it. Must be willing to play the part of the Great
Pumpkin now and then.

=item PV

A “X<PV (pointer value)>X<pointer value (PV)>pointer value”, which is Perl
Internals Talk for a C<char*>.

=back

=head2 Q

=over 4

=item qualified

Possessing a X<qualified (term)>complete name. The symbol C<$Ent::moot> is
qualified; C<$moot> is unqualified. A fully qualified filename is specified
from the top-level directory.

=item quantifier

A X<quantifiers, about>component of a B<regular expression> specifying how
many times the foregoing B<atom> may occur.

=back

=head2 R

=over 4

=item race condition

A X<race conditions, defined>race condition exists when the result of
several interrelated events depends on the ordering of those events, but
that order cannot be guaranteed due to nondeterministic timing effects. If
two or more programs, or parts of the same program, try to go through the
same series of events, one might interrupt the work of the other. This is a
good way to find an B<exploit>.

=item readable

With X<readable (term)>respect to files, one that has the proper permission
bit set to let you access the file. With respect to computer programs, one
that’s written well enough that someone has a chance of figuring out what
it’s trying to do.

=item reaping

The last X<reaping zombie processes>rites performed by a parent B<process>
on behalf of a deceased child process so that it doesn’t remain a
B<zombie>.  See the C<wait> and C<waitpid> function calls.

=item record

A set of X<records, defined>related data values in a B<file> or B<stream>,
often associated with a unique B<key> field. In Unix, often commensurate
with a B<line>, or a blank-line–terminated set of lines (a “paragraph”).
Each line of the I</etc/passwd> file is a record, keyed on login name,
containing information about that user.

=item recursion

The art of X<recursion, defined>defining something (at least partly) in
terms of itself, which is a naughty no-no in dictionaries but often works
out okay in computer programs if you’re careful not to recurse forever
(which is like an infinite loop with more spectacular failure modes).

=item reference

Where you X<references, about>look to find a pointer to information
somewhere else. (See B<indirection>.) References come in two flavors:
B<symbolic references> and B<hard references>.

=item referent

Whatever a X<referents, defined>reference refers to, which may or may not
have a name. Common types of referents include scalars, arrays, hashes, and
subroutines.

=item regex

See B<regular expression>.

=item regular expression

A single X<regular expressions, defined>entity with various
interpretations, like an elephant. To a computer scientist, it’s a grammar
for a little language in which some strings are legal and others aren’t. To
normal people, it’s a pattern you can use to find what you’re looking for
when it varies from case to case. Perl’s regular expressions are far from
regular in the theoretical sense, but in regular use they work quite well.
Here’s a regular expression: C</Oh s.*t./>. This will match strings like
“C<Oh say can you see by the dawn's early light>” and “C<Oh sit!>”. See
Camel chapter 5, “Pattern Matching”.

=item regular expression modifier

An option on a X<regular expression modifiers>X<modifiers, regular
expression>pattern or substitution, such as C</i> to render the pattern
case- insensitive.

=item regular file

A B<file> that’s X<regular files>X<files, regular>not a B<directory>, a
B<device>, a named B<pipe> or B<socket>, or a B<symbolic link>. Perl uses
the C<–f> file test operator to identify regular files. Sometimes called a
“plain” file.

=item relational operator

An B<operator> that X<relational operators>says whether a particular
ordering relationship is B<true> about a pair of B<operands>. Perl has both
numeric and string relational operators. See B<collating sequence>.

=item reserved words

A word with a X<reserved words>X<keywords (term)>specific, built-in meaning
to a B<compiler>, such as C<if> or C<delete>. In many languages (not Perl),
it’s illegal to use reserved words to name anything else. (Which is why
they’re reserved, after all.) In Perl, you just can’t use them to name
B<labels> or B<filehandles>. Also called “keywords”.

=item return value

The B<value> produced X<return values>X<values, return>by a B<subroutine>
or B<expression> when evaluated. In Perl, a return value may be either a
B<list> or a B<scalar>.

=item RFC

Request For Comment, X<Request For Comment (RFC)>X<RFC (Request For
Comment)>which despite the timid connotations is the name of a series of
important standards documents.

=item right shift

A B<bit shift> X<right shift (E<gt>E<gt>) bit operator>X<bit–shift
operators, right shift>X<E<gt>E<gt> (right shift) bit operator>that divides
a number by some power of 2.

=item role

A name X<roles (term)>for a concrete set of behaviors. A role is a way to
add behavior to a class without inheritance.

=item root

The X<root (term)>superuser (C<UID> == 0). Also the top-level directory of
the filesystem.

=item RTFM

What X<RTFM acronym>you are told when someone thinks you should Read The
Fine Manual.

=item run phase

Any X<run phase, defined>time after Perl starts running your main program.
See also B<compile phase>. Run phase is mostly spent in B<runtime> but may
also be spent in B<compile time> when C<require>, C<do> I<C<FILE>>, or
C<eval> I<C<STRING>> operators are executed, or when a substitution uses
the C</ee> modifier.

=item runtime

The time X<runtime (term), defined>when Perl is actually doing what your
code says to do, as opposed to the earlier period of time when it was
trying to figure out whether what you said made any sense whatsoever, which
is B<compile time>.

=item runtime pattern

A X<runtime patterns>X<patterns, runtime>pattern that contains one or more
variables to be interpolated before parsing the pattern as a B<regular
expression>, and that therefore cannot be analyzed at compile time, but
must be reanalyzed each time the pattern match operator is evaluated.
Runtime patterns are useful but expensive.

=item RV

A X<Reference Value (RV)>X<RV (Reference Value)>recreational vehicle, not
to be confused with vehicular recreation. RV also means an internal
Reference Value of the type a B<scalar> can hold. See also B<IV> and B<NV>
if you’re not confused yet.

=item rvalue

A B<value> that X<rvalue (term)>X<values, rvalue>you might find on the
right side of an B<assignment>. See also B<lvalue>.

=back

=head2 S

=over 4

=item sandbox

A X<sandbox, defined>walled off area that’s not supposed to affect beyond
its walls. You let kids play in the sandbox instead of running in the road.
See Camel chapter 20, “Security”.

=item scalar

A X<scalars, defined>simple, singular value; a number, B<string>, or
B<reference>.

=item scalar context

The X<scalar context, about>X<context, scalar>situation in which an
B<expression> is expected by its surroundings (the code calling it) to
return a single B<value> rather than a B<list> of values. See also
B<context> and B<list context>. A scalar context sometimes imposes
additional constraints on the return value—see B<string context> and
B<numeric context>. Sometimes we talk about a B<Boolean context> inside
conditionals, but this imposes no additional constraints, since any scalar
value, whether numeric or B<string>, is already true or false.

=item scalar literal

A X<scalar literals>X<literals, scalar>number or quoted B<string>—an actual
B<value> in the text of your program, as opposed to a B<variable>.

=item scalar value

A X<scalar values, about>X<values, scalar>X<SV>value that happens to be a
B<scalar> as opposed to a B<list>.

=item scalar variable

A B<variable> X<scalar variables, defined>X<variables, scalar>prefixed with
C<$> that holds a single value.

=item scope

From X<scopes, defined>how far away you can see a variable, looking through
one. Perl has two visibility mechanisms. It does B<dynamic scoping> of
C<local> B<variables>, meaning that the rest of the B<block>, and any
B<subroutines> that are called by the rest of the block, can see the
variables that are local to the block. Perl does B<lexical scoping> of
C<my> variables, meaning that the rest of the block can see the variable,
but other subroutines called by the block I<cannot> see the variable.

=item scratchpad

The X<scratchpads>area in which a particular invocation of a particular
file or subroutine keeps some of its temporary values, including any
lexically scoped variables.

=item script

A X<scripts (term)>X<programs, defined>text B<file> that is a program
intended to be B<executed> directly rather than B<compiled> to another form
of file before B<execution>.

Also, in the context of B<Unicode>, a writing system for a particular
language or group of languages, such as Greek, Bengali, or Tengwar.

=item script kiddie

A B<cracker> X<script kiddie>who is not a B<hacker> but knows just enough
to run canned scripts. A B<cargo-cult> programmer.

=item sed

A venerable Stream EDitor X<sed (Stream EDitor)>X<Stream EDitor (sed)>from
which Perl derives some of its ideas.

=item semaphore

A fancy X<semaphore>kind of interlock that prevents multiple B<threads> or
B<processes> from using up the same resources simultaneously.

=item separator

A B<character> X<separators>X<characters, separators>X<strings,
separators>or B<string> that keeps two surrounding strings from being
confused with each other. The C<split> function X<split function,
separators and>works on separators. Not to be confused with B<delimiters>
or B<terminators>. The “or” in the previous sentence separated the two
alternatives.

=item serialization

Putting a X<serialization>X<marshalling (term)>fancy B<data structure> into
linear order so that it can be stored as a B<string> in a disk file or
database, or sent through a B<pipe>. Also called marshalling.

=item server

In networking, X<servers, defined>X<processes, server>a B<process> that
either advertises a B<service> or just hangs around at a known location and
waits for B<clients> who need service to get in touch with it.

=item service

Something X<services (term)>you do for someone else to make them happy,
like giving them the time of day (or of their life). On some machines,
well-known services are listed by theX<getservent function> C<getservent>
function.

=item setgid

Same as B<setuid>, X<setgid program, about>only having to do with giving
away B<group> privileges.

=item setuid

Said of a program X<setuid program, about>that runs with the privileges of
its B<owner> rather than (as is usually the case) the privileges of whoever
is running it. Also describes the bit in the mode word (B<permission bits>)
that controls the feature. This bit must be explicitly set by the owner to
enable this feature, and the program must be carefully written not to give
away more privileges than it ought to.

=item shared memory

A piece of B<memory> X<shared memory>X<memory, shared>accessible by two
different B<processes> who otherwise would not see each other’s memory.

=item shebang

Irish for the X<shebang (term)>whole McGillicuddy. In Perl culture, a
portmanteau of “sharp” and “bang”, meaning the C<#!> sequence that tells
the system where to find the interpreter.

=item shell

A B<command>-X<shell program, defined>line B<interpreter>. The program that
interactively gives you a prompt, accepts one or more B<lines> of input,
and executes the programs you mentioned, feeding each of them their proper
B<arguments> and input data. Shells can also execute scripts containing
such commands. Under Unix, typical shells include the Bourne shell
(I</bin/sh>), the C shell (I</bin/csh>), and the Korn shell (I</bin/ksh>).
Perl is not strictly a shell because it’s not interactive (although Perl
programs can be interactive).

=item side effects

Something extra X<side effects>that happens when you evaluate an
B<expression>. Nowadays it can refer to almost anything. For example,
evaluating a simple assignment statement typically has the “side effect” of
assigning a value to a variable. (And you thought assigning the value was
your primary intent in the first place!) Likewise, assigning a value to the
special variable C<$|> (C<$AUTOFLUSH>) has the side effect of forcing a
flush after every C<write> or C<print> on the currently selected
filehandle.

=item sigil

A glyph X<sigils, defined>used in magic. Or, for Perl, the symbol in front
of a variable name, such as C<$>, C<@>, and C<%>.

=item signal

A bolt X<signals and signal handling, about>out of the blue; that is, an
event triggered by the B<operating system>, probably when you’re least
expecting it.

=item signal handler

A B<subroutine> that, X<handlers, signal>instead of being content to be
called in the normal fashion, sits around waiting for a bolt out of the
blue before it will deign to B<execute>. Under Perl, bolts out of the blue
are called signals, and you send them with the C<kill> built-in. See the
C<%SIG> hash in Camel chapter 25, “Special Names” and the section “Signals”
in Camel chapter 15, “Interprocess Communication”.

=item single inheritance

The features X<single inheritance>X<inheritance, single>you got from your
mother, if she told you that you don’t have a father. (See also
B<inheritance> and B<multiple inheritance>.) In computer languages, the
idea that B<classes> reproduce asexually so that a given class can only
have one direct ancestor or B<base class>. Perl supplies no such
restriction, though you may certainly program Perl that way if you like.

=item slice

A selection X<slices of elements>X<elements, slices of>of any number of
B<elements> from a B<list>, B<array>, or B<hash>.

=item slurp

To read an X<slurp (term)>entire B<file> into a B<string> in one operation.

=item socket

An endpoint for X<sockets, defined>network communication among multiple
B<processes> that works much like a telephone or a post office box. The
most important thing about a socket is its B<network address> (like a phone
number). Different kinds of sockets have different kinds of addresses—some
look like filenames, and some don’t.

=item soft reference

SeeX<soft references>X<references, soft> B<symbolic reference>.

=item source filter

A special X<source filters>X<filters, source>kind of B<module> that does
B<preprocessing> on your script just before it gets to the B<tokener>.

=item stack

A X<stacks, defined>device you can put things on the top of, and later take
them back off in the opposite order in which you put them on. See B<LIFO>.

=item standard

Included X<standard (term)>in the official Perl distribution, as in a
standard module, a standard tool, or a standard Perl B<manpage>.

=item standard error

The default output B<stream> for nasty remarks that don’t belong in
B<standard output>. Represented within a Perl program by theX<STDERR
filehandle, about> output>  B<filehandle> C<STDERR>. You can use this
stream explicitly, but the C<die> and C<warn> built-ins write to your
standard error stream automatically (unless trapped or otherwise
intercepted).

=item standard input

The X<STDIN filehandle, about>default input B<stream> for your program,
which if possible shouldn’t care where its data is coming from. Represented
within a Perl program by the B<filehandle> C<STDIN>.

=item standard I/O

A X<standard I/O>X<I/O (Input/Output), standard>X<Input/Output (I/O),
standard>X<STDIO filehandle>standard C library for doing B<buffered> input
and output to the B<operating system>. (The “standard” of standard I/O is
at most marginally related to the “standard” of standard input and output.)
In general, Perl relies on whatever implementation of standard I/O a given
operating system supplies, so the buffering characteristics of a Perl
program on one machine may not exactly match those on another machine.
Normally this only influences efficiency, not semantics. If your standard
I/O package is doing block buffering and you want it to B<flush> the buffer
more often, just set the C<$|> variable to a true value.

=item Standard Library

Everything X<Standard Perl Library, about>that comes with the official
I<perl> distribution. Some vendor versions of I<perl> change their
distributions, leaving out some parts or including extras. See also
B<dual-lived>.

=item standard output

The X<STDOUT filehandle, about>default output B<stream> for your program,
which if possible shouldn’t care where its data is going. Represented
within a Perl program by the B<filehandle> C<STDOUT>.

=item statement

A B<command> to X<statements, about>the computer about what to do next,
like a step in a recipe: “Add marmalade to batter and mix until mixed.” A
statement is distinguished from a B<declaration>, which doesn’t tell the
computer to do anything, but just to learn something.

=item statement modifier

A B<conditional> X<statement modifiers, about>X<modifiers, statement>or
B<loop> that you put after the B<statement> instead of before, if you know
what we mean.

=item static

Varying X<static (term)>slowly compared to something else. (Unfortunately,
everything is relatively stable compared to something else, except for
certain elementary particles, and we’re not so sure about them.) In
computers, where things are supposed to vary rapidly, “static” has a
derogatory connotation, indicating a slightly dysfunctional B<variable>,
B<subroutine>, or B<method>. In Perl culture, the word is politely avoided.

If you’re a C or C++ programmer, you might be looking for Perl’s C<state>
keyword.

=item static method

No such X<static methods>X<methods, static>thing. See B<class method>.

=item static scoping

No such thing. See B<lexical scoping>.

=item static variable

No such X<static variables>X<variables, static>thing. Just use a B<lexical
variable> in a scope larger than your B<subroutine>, or declare it with
C<state> instead of with C<my>.

=item stat structure

A special X<stat structure>X<data structures, stat structure>internal spot
in which Perl keeps the information about the last B<file> on which you
requested information.

=item status

The B<value> X<status value>X<values, status>X<exit status>returned to the
parent B<process> when one of its child processes dies. This value is
placed in the special variable C<$?>. Its upper eight B<bits> are the exit
status of the defunct process, and its lower eight bits identify the signal
(if any) that the process died from. On Unix systems, this status value is
the same as the status word returned by I<wait>(2). See C<system> in Camel
chapter 27, “Functions”.

=item STDERR

See B<standard error>.

=item STDIN

See B<standard input>.

=item STDIO

See B<standard I/O>.

=item STDOUT

See B<standard output>.

=item stream

A flow X<streaming data>X<processes, streaming data>of data into or out of
a process as a steady sequence of bytes or characters, without the
appearance of being broken up into packets. This is a kind of
B<interface>—the underlying B<implementation> may well break your data up
into separate packets for delivery, but this is hidden from you.

=item string

A sequence X<strings, defined>of characters such as “He said !@#*&%@#*?!”.
A string does not have to be entirely printable.

=item string context

The situation X<string context>X<context, string>in which an expression is
expected by its surroundings (the code calling it) to return a B<string>.
See also B<context> and B<numeric context>.

=item stringification

The process X<stringification>of producing a B<string> representation of an
abstract object.

=item struct

C keyword X<struct keyword>introducing a structure definition or name.

=item structure

SeeX<structures> B<data structure>.

=item subclass

See B<derived class>.

=item subpattern

A X<subpatterns, defined>component of a B<regular expression> pattern.

=item subroutine

A X<subroutines, defined>named or otherwise accessible piece of program
that can be invoked from elsewhere in the program in order to accomplish
some subgoal of the program. A subroutine is often parameterized to
accomplish different but related things depending on its input
B<arguments>. If the subroutine returns a meaningful B<value>, it is also
called a B<function>.

=item subscript

A B<value> X<subscripts>that indicates the position of a particular
B<array> B<element> in an array.

=item substitution

Changing X<substitution (s///) operator, about>X<strings, substitution
in>X<s/// (substitution) operator, about>parts of a string via the C<s///>
operator. (We avoid use of this term to mean B<variable interpolation>.)

=item substring

A portion of a B<string>, X<substrings (term)>starting at a certain
B<character> position (B<offset>) and proceeding for a certain number of
characters.

=item superclass

See B<base class>.

=item superuser

The X<superusers>person whom the B<operating system> will let do almost
anything. Typically your system administrator or someone pretending to be
your system administrator. On Unix systems, the B<root> user. On Windows
systems, usually the Administrator user.

=item SV

Short X<scalar values, about>X<values, scalar>for “scalar value”. But
within the Perl interpreter, every B<referent> is treated as a member of a
class derived from SV, in an object-oriented sort of way. Every B<value>
inside Perl is passed around as a C language C<SV*> pointer. The SV
B<struct> knows its own “referent type”, and the code is smart enough (we
hope) not to try to call a B<hash> function on a B<subroutine>.

=item switch

An X<switches, about>X<switches>option you give on a command line to
influence the way your program works, usually introduced with a minus sign.
The word is also used as a nickname for a B<switch statement>.

=item switch cluster

The X<switch clusters>X<clusters, switch>combination of multiple command-
line switches (I<e.g.>, C<–a –b –c>) into one switch (I<e.g.>, C<–abc>).
Any switch with an additional B<argument> must be the last switch in a
cluster.

=item switch statement

A X<switch statement>X<statements, switch>program technique that lets you
evaluate an B<expression> and then, based on the value of the expression,
do a multiway branch to the appropriate piece of code for that value. Also
called a “case structure”, named after the similar Pascal construct. Most
switch statements in Perl are spelled C<given>. See “The C<given>
statement” in Camel chapter 4, “Statements and Declarations”.

=item symbol

Generally, X<symbols>X<symbols>any B<token> or B<metasymbol>. Often used
more specifically to mean the sort of name you might find in a B<symbol
table>.

=item symbolic debugger

A program X<symbolic debugger>X<debugger, about>that lets you step through
the B<execution> of your program, stopping or printing things out here and
there to see whether anything has gone wrong, and, if so, what. The
“symbolic” part just means that you can talk to the debugger using the same
symbols with which your program is written.

=item symbolic link

An alternate X<symbolic links>X<links, symbolic>filename that points to the
real B<filename>, which in turn points to the real B<file>. Whenever the
B<operating system> is trying to parse a B<pathname> containing a symbolic
link, it merely substitutes the new name and continues parsing.

=item symbolic reference

A variable X<symbolic references>X<references, symbolic>whose value is the
name of another variable or subroutine. By B<dereferencing> the first
variable, you can get at the second one. Symbolic references are illegal
under C<use strict "refs">.

=item symbol table

Where X<symbol tables, about>a B<compiler> remembers symbols. A program
like Perl must somehow remember all the names of all the B<variables>,
B<filehandles>, and B<subroutines> you’ve used. It does this by placing the
names in a symbol table, which is implemented in Perl using a B<hash
table>. There is a separate symbol table for each B<package> to give each
package its own B<namespace>.

=item synchronous

Programming X<synchronous (term)>in which the orderly sequence of events
can be determined; that is, when things happen one after the other, not at
the same time.

=item syntactic sugar

An X<syntactic sugar>alternative way of writing something more easily; a
shortcut.

=item syntax

From X<syntax, about>Greek σύνταξις, “with-arrangement”. How things
(particularly symbols) are put together with each other.

=item syntax tree

An internal X<syntax tree>representation of your program wherein
lower-level B<constructs> dangle off the higher-level constructs enclosing
them.

=item syscall

A B<function> X<syscall function, about>call directly to the B<operating
system>. Many of the important subroutines and functions you use aren’t
direct system calls, but are built up in one or more layers above the
system call level. In general, Perl programmers don’t need to worry about
the distinction. However, if you do happen to know which Perl functions are
really syscalls, you can predict which of these will set the C<$!>
(C<$ERRNO>) variable on failure. Unfortunately, beginning programmers often
confusingly employ the term “system call” to mean what happens when you
call the Perl C<system> function, which actually involves many syscalls. To
avoid any confusion, we nearly always say “syscall” for something you could
call indirectly via Perl’s C<syscall> function, and never for something you
would call with Perl’s C<system> function.

=back

=head2 T

=over 4

=item taint checks

The X<taint checks, about>special bookkeeping Perl does to track the flow
of external data through your program and disallow their use in system
commands.

=item tainted

Said of X<tainted data, about>data derived from the grubby hands of a user,
and thus unsafe for a secure program to rely on. Perl does taint checks if
you run a B<setuid> (or B<setgid>) program, or if you use the C<–T> switch.

=item taint mode

Running X<taint mode>under the C<–T> switch, marking all external data as
suspect and refusing to use it with system commands. See Camel chapter 20,
“Security”.

=item TCP

Short for X<TCP (Transmission Control Protocol)>X<Transmission Control
Protocol (TCP)>Transmission Control Protocol. A protocol wrapped around the
Internet Protocol to make an unreliable packet transmission mechanism
appear to the application program to be a reliable B<stream> of bytes.
(Usually.)

=item term

Short for X<terms, defined>a “terminal”—that is, a leaf node of a B<syntax
tree>. A thing that functions grammatically as an B<operand> for the
operators in an expression.

=item terminator

A B<character> X<terminators (term)>X<characters, terminators>X<strings,
terminators in>or B<string> that marks the end of another string. The C<$/>
variable contains the string that terminates a C<readline> operation, which
C<chomp> deletes from the end. Not to be confused with B<delimiters> or
B<separators>. The period at the end of this sentence is a terminator.

=item ternary

An B<operator> X<ternary operators>taking three B<operands>. Sometimes
pronounced B<trinary>.

=item text

A B<string> or B<file> X<text, defined>X<strings, text>X<files,
text>X<text>containing primarily printable characters.

=item thread

Like a X<threads (term)>forked process, but without B<fork>’s inherent
memory protection. A thread is lighter weight than a full process, in that
a process could have multiple threads running around in it, all fighting
over the same process’s memory space unless steps are taken to protect
threads from one another.

=item tie

The bond X<tied variables, about>between a magical variable and its
implementation class. See the C<tie> function in Camel chapter 27,
“Functions” and Camel chapter 14, “Tied Variables”.

=item titlecase

The case X<titlecase characters>X<characters, titlecase>used for capitals
that are followed by lowercase characters instead of by more capitals.
Sometimes called sentence case or headline case. English doesn’t use
Unicode titlecase, but casing rules for English titles are more complicated
than simply capitalizing each word’s first character.

=item TMTOWTDI

There’s More Than One Way To Do It, the Perl MottoX<TMTOWTDI acronym>. The
notion that there can be more than one valid path to solving a programming
problem in context. (This doesn’t mean that more ways are always better or
that all possible paths are equally desirable—just that there need not be
One True Way.)

=item token

A morpheme X<tokens, defined>in a programming language, the smallest unit
of text with semantic significance.

=item tokener

A module that X<tokeners, defined>breaks a program text into a sequence of
B<tokens> for later analysis by a parser.

=item tokenizing

Splitting up a X<tokenizing>program text into B<tokens>. Also known as
“lexing”, in which case you get “lexemes” instead of tokens.

=item toolbox approach

The notion that, X<toolbox approach>with a complete set of simple tools
that work well together, you can build almost anything you want. Which is
fine if you’re assembling a tricycle, but if you’re building a
defranishizing comboflux regurgalator, you really want your own machine
shop in which to build special tools. Perl is sort of a machine shop.

=item topic

The thing you’re X<topics (term)>working on. Structures like
C<while(E<lt>E<gt>)>, C<for>, C<foreach>, and C<given> set the topic for
you by assigning to C<$_>, the default (I<topic>) variable.

=item transliterate

To turn one X<tr/// (transliteration) operator, about>X<strings,
transliteration of>X<transliteration (tr///) operator, about>string
representation into another by mapping each character of the source string
to its corresponding character in the result string. Not to be confused
with translation: for example, Greek I<πολύχρωμος> transliterates into
I<polychromos> but translates into I<many-colored>. See the C<tr///>
operator in Camel chapter 5, “Pattern Matching”.

=item trigger

An event X<triggers (term)>that causes a B<handler> to be run.

=item trinary

Not a X<trinary operators>stellar system with three stars, but an
B<operator> taking three B<operands>. Sometimes pronounced B<ternary>.

=item troff

A venerable X<troff language>typesetting language from which Perl derives
the name of its C<$%> variable and which is secretly used in the production
of Camel books.

=item true

Any X<true values>X<values, true>scalar value that doesn’t evaluate to 0 or
C<"">.

=item truncating

Emptying a X<truncate function>X<files, truncating>file of existing
contents, either automatically when opening a file for writing or
explicitly via the C<truncate> function.

=item type

SeeX<type> B<data type> and B<class>.

=item type casting

Converting X<type casting>data from one type to another. C permits this.
Perl does not need it. Nor want it.

=item typedef

A type X<typedef>definition in the C and C++ languages.

=item typed lexical

A B<lexical variable> X<typed lexicals>X<lexical variables, typed
lexicals>X<variables, variable> lexical>that is declared with a B<class>
type: C<my Pony $bill>.

=item typeglob

Use of X<typeglobs, defined>a single identifier, prefixed with C<*>. For
example, C<*name> stands for any or all of C<$name>, C<@name>, C<%name>,
C<&name>, or just C<name>. How you use it determines whether it is
interpreted as all or only one of them. See “Typeglobs and Filehandles” in
Camel chapter 2, “Bits and Pieces”.

=item typemap

A description of X<typemap>how C types may be transformed to and from Perl
types within an B<extension> module written in B<XS>.

=back

=head2 U

=over 4

=item UDP

User Datagram Protocol, the X<User Datagram Protocol (UDP)>X<UDP (User
Datagram Protocol)>X<datagrams, UDP support>typical way to send
B<datagrams> over the Internet.

=item UID

A user ID. X<UID (user ID)>X<user ID (UID)>Often used in the context of
B<file> or B<process> ownership.

=item umask

A X<umask function>mask of those B<permission bits> that should be forced
off when creating files or directories, in order to establish a policy of
whom you’ll ordinarily deny access to. See the C<umask> function.

=item unary operator

An X<unary operators, about>operator with only one B<operand>, like C<!> or
C<chdir>. Unary operators are usually prefix operators; that is, they
precede their operand. The C<++> and C<––> operators can be either prefix
or postfix. (Their position I<does> change their meanings.)

=item Unicode

A character set X<Unicode, about>comprising all the major character sets of
the world, more or less. See L<http://www.unicode.org>.

=item Unix

A very large X<Unix language>and constantly evolving language with several
alternative and largely incompatible syntaxes, in which anyone can define
anything any way they choose, and usually do. Speakers of this language
think it’s easy to learn because it’s so easily twisted to one’s own ends,
but dialectical differences make tribal intercommunication nearly
impossible, and travelers are often reduced to a pidgin-like subset of the
language. To be universally understood, a Unix shell programmer must spend
years of study in the art. Many have abandoned this discipline and now
communicate via an Esperanto-like language called Perl.

In ancient times, Unix was also used to refer to some code that a couple of
people at Bell Labs wrote to make use of a PDP-7 computer that wasn’t doing
much of anything else at the time.

=item uppercase

In Unicode, X<uppercase characters>X<characters, uppercase>not just
characters with the General Category of Uppercase Letter, but any character
with the Uppercase property, including some Letter Numbers and Symbols. Not
to be confused with B<titlecase>.

=back

=head2 V

=over 4

=item value

An actual piece X<values, defined>of data, in contrast to all the
variables, references, keys, indices, operators, and whatnot that you need
to access the value.

=item variable

A named storage X<variables, defined>X<variables>location that can hold any
of various kinds of B<value>, as your program sees fit.

=item variable interpolation

TheX<variable interpolation>X<interpolation, variable> B<interpolation> of
a scalar or array variable into a string.

=item variadic

Said of X<variadic (term)>a B<function> that happily receives an
indeterminate number of B<actual arguments>.

=item vector

Mathematical X<vectors>jargon for a list of B<scalar values>.

=item virtual

Providing the X<virtual (term)>appearance of something without the reality,
as in: virtual memory is not real memory. (See also B<memory>.) The
opposite of “virtual” is “transparent”, which means providing the reality
of something without the appearance, as in: Perl handles the
variable-length UTF‑8 character encoding transparently.

=item void context

A form X<void context>X<context, void>of B<scalar context> in which an
B<expression> is not expected to return any B<value> at all and is
evaluated for its B<side effects> alone.

=item v-string

A “version” or “vector”X<v–strings>X<strings, v–strings> B<string>
specified with a C<v> followed by a series of decimal integers in dot
notation, for instance, C<v1.20.300.4000>. Each number turns into a
B<character> with the specified ordinal value. (The C<v> is optional when
there are at least three integers.)

=back

=head2 W

=over 4

=item warning

A message X<warning messages>X<STDERR filehandle, warning messages
and>printed to the C<STDERR> stream to the effect that something might be
wrong but isn’t worth blowing up over. See C<warn> in Camel chapter 27,
“Functions” and the C<warnings> pragma in Camel chapter 28, “Pragmantic
Modules”.

=item watch expression

An expression which, X<watch expression>X<expressions, watch>when its value
changes, causes a breakpoint in the Perl debugger.

=item weak reference

A X<weak references>X<references, weak>reference that doesn’t get counted
normally. When all the normal references to data disappear, the data
disappears. These are useful for circular references that would never
disappear otherwise.

=item whitespace

A B<character> X<whitespace characters>X<characters, whitespace>that moves
your cursor but doesn’t otherwise put anything on your screen. Typically
refers to any of: space, tab, line feed, carriage return, or form feed. In
Unicode, matches many other characters that Unicode considers whitespace,
including the ɴ-ʙʀ .

=item word

In normal “computerese”, the X<words (term)>piece of data of the size most
efficiently handled by your computer, typically 32 bits or so, give or take a
few powers of 2. In Perl culture, it more often refers to an alphanumeric
B<identifier> (including underscores), or to a string of nonwhitespace
B<characters> bounded by whitespace or string boundaries.

=item working directory

Your X<working directory>X<directories, working>current B<directory>, from
which relative pathnames are interpreted by the B<operating system>. The
operating system knows your current directory because you told it with a
C<chdir>, or because you started out in the place where your parent
B<process> was when you were born.

=item wrapper

A program X<wrappers (term)>or subroutine that runs some other program or
subroutine for you, modifying some of its input or output to better suit
your purposes.

=item WYSIWYG

What X<WYSIWYG acronym>You See Is What You Get. Usually used when something
that appears on the screen matches how it will eventually look, like Perl’s
C<format> declarations. Also used to mean the opposite of magic because
everything works exactly as it appears, as in the three- argument form of
C<open>.

=back

=head2 X

=over 4

=item XS

An X<XS (eXternal Subroutine)>X<eXternal Subroutine (XS)>extraordinarily
exported, expeditiously excellent, expressly eXternal Subroutine, executed
in existing C or C++ or in an exciting extension language called
(exasperatingly) XS.

=item XSUB

An X<XSUB (term)>external B<subroutine> defined in B<XS>.

=back

=head2 Y

=over 4

=item yacc

Yet X<yacc acronym>Another Compiler Compiler. A parser generator without
which Perl probably would not have existed. See the file I<perly.y> in the
Perl source distribution.

=back

=head2 Z

=over 4

=item zero width

A X<zero–width assertions>X<subpatterns, zero–width assertions>X<assertions
(in regexes), zero–width>subpattern B<assertion> matching the B<null
string> between B<characters>.

=item zombie

A process X<zombie processes>X<processes, zombie>that has died (exited) but
whose parent has not yet received proper notification of its demise by
virtue of having called C<wait> or C<waitpid>. If you C<fork>, you must
clean up after your child processes when they exit; otherwise, the process
table will fill up and your system administrator will Not Be Happy with
you.

=back

=head1 AUTHOR AND COPYRIGHT

Based on the Glossary of I<Programming Perl>, Fourth Edition,
by Tom Christiansen, brian d foy, Larry Wall, & Jon Orwant.
Copyright (c) 2000, 1996, 1991, 2012 O'Reilly Media, Inc.
This document may be distributed under the same terms as Perl itself.