Removed unnecessary pointers checks

[perl5.git] / pod / perlglossary.pod

=head1 NAME

perlglossary - Perl Glossary

=head1 DESCRIPTION

A glossary of terms (technical and otherwise) used in the Perl documentation.
Other useful sources include the Free On-Line Dictionary of Computing
L<http://foldoc.doc.ic.ac.uk/foldoc/index.html>, the Jargon File
L<http://catb.org/~esr/jargon/>, and Wikipedia L<http://www.wikipedia.org/>.

=head2 A

=over 4

=item accessor methods

A L</method> used to indirectly inspect or update an L</object>'s
state (its L<instance variables|/instance variable>).

=item actual arguments

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

=item address operator

Some 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 L</hard
reference>, which is much safer than a memory address.

=item algorithm

A well-defined sequence of steps, clearly enough explained that even a
computer could do them.

=item alias

A 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<$_> variable for L<map|perlfunc/map> or L<grep|perlfunc/grep>
operators, in C<$a> and C<$b> during L<sort|perlfunc/sort>'s
comparison function, and in each element of C<@_> for the L</actual
arguments> of a subroutine call.  Permanent aliases are explicitly
created in L<packages|/package> by L<importing|/import> symbols or by
assignment to L<typeglobs|/typeglob>.  Lexically scoped aliases for
package variables are explicitly created by the L<our|perlfunc/our>
declaration.

=item alternatives

A 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 L</Boolean> expressions are separated
with either C<||> or C<or>.

=item anonymous

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

=item architecture

The kind of 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
compiled into machine code.  See also L</platform> and L</operating
system>.

=item argument

A piece of data supplied to a L<program|/executable file>,
L</subroutine>, L</function>, or L</method> to tell it what it's
supposed to do.  Also called a "parameter".

=item ARGV

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

=item arithmetical operator

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

=item array

An ordered sequence of L<values|/value>, stored such that you can
easily access any of the values using an integer L</subscript>
that specifies the value's L</offset> in the sequence.

=item array context

An archaic expression for what is more correctly referred to as
L</list context>.

=item ASCII

The 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 L</Unicode>.

=item assertion

A component of a L</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 L</zero width> assertion.

=item assignment

An L</operator> whose assigned mission in life is to change the value
of a L</variable>.

=item assignment operator

Either a regular L</assignment>, or a compound L</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 L</hash>.  Please.

=item associativity

Determines whether you do the left L</operator> first or the right
L</operator> first when you have "A L</operator> B L</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 L<perlop> for a list of operators and their associativity.

=item asynchronous

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

=item atomic operation

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

=item attribute

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

=item autogeneration

A feature of L</operator overloading> of L<objects|/object>, whereby
the behavior of certain L<operators|/operator> 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 L<perlop>.

=item autoincrement

To 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 load on demand.  (Also called "lazy" loading.)  Specifically, to
call an L<AUTOLOAD|perlsub/Autoloading> subroutine on behalf of an
undefined subroutine.

=item autosplit

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

=item autovivification

A Greco-Roman word meaning "to bring oneself to life".  In Perl,
storage locations (L<lvalues|/lvalue>) spontaneously generate
themselves as needed, including the creation of any L</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 for "array value", which refers to one of Perl's internal data
types that holds an L</array>.  The L</AV> type is a subclass of
L</SV>.

=item awk

Descriptive 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 substring L<captured|/capturing> by a subpattern within
unadorned parentheses in a L</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 dynamic scope.

=item backtracking

The 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 L</regular expression>, and
its earlier attempts don't pan out.  See L<perlre/Backtracking>.

=item backward compatibility

Means 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 sufficiently ambiguous to be deemed illegal under L<use strict
'subs'|strict/strict subs>.  In the absence of that stricture, a
bareword is treated as if quotes were around it.

=item base class

A generic L</object> type; that is, a L</class> from which other, more
specific classes are derived genetically by L</inheritance>.  Also
called a "superclass" by people who respect their ancestors.

=item big-endian

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

=item binary

Having to do with numbers represented in base 2.  That means there's
basically two numbers, 0 and 1.  Also used to describe a "non-text
file", presumably because such a file makes full use of all the binary
bits in its bytes.  With the advent of L</Unicode>, this distinction,
already suspect, loses even more of its meaning.

=item binary operator

An L</operator> that takes two L<operands|/operand>.

=item bind

To assign a specific L</network address> to a L</socket>.

=item bit

An integer in the range from 0 to 1, inclusive.  The smallest possible
unit of information storage.  An eighth of a L</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 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 sequence of L<bits|/bit> that is actually being thought of as a
sequence of bits, for once.

=item bless

In 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 L</referent> so that it can
function as an L</object>, such as a WebCruncher object.  See
L<perlfunc/"bless">.

=item block

What a L</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 L</operating system> likes to
deal with (normally a power of two 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 syntactic construct consisting of a sequence of Perl
L<statements|/statement> that is delimited by braces.  The C<if> and
C<while> statements are defined in terms of L<BLOCKs|/BLOCK>, for instance.
Sometimes we also say "block" to mean a lexical scope; that is, a
sequence of statements that act like a L</BLOCK>, such as within an
L<eval|perlfunc/eval> or a file, even though the statements aren't
delimited by braces.

=item block buffering

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

=item Boolean

A value that is either L</true> or L</false>.

=item Boolean context

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

=item breakpoint

A spot in your program where you've told the debugger to stop
L<execution|/execute> so you can poke around and see whether anything
is wrong yet.

=item broadcast

To send a L</datagram> to multiple destinations simultaneously.

=item BSD

A psychoactive drug, popular in the 80s, probably developed at
U. C. 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 location in a L</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 temporary holding location for data.  L<Block buffering|/block
buffering> means that the data is passed on to its destination
whenever the buffer is full.  L<Line buffering|/line buffering> means
that it's passed on whenever a complete line is received.  L<Command
buffering|/command buffering> means that it's passed every time you do
a L<print|perlfunc/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 L</function> that is predefined in the language.  Even when hidden
by L</overriding>, you can always get at a built-in function by
L<qualifying|/qualified> its name with the C<CORE::> pseudo-package.

=item bundle

A group of related modules on L</CPAN>.  (Also, sometimes refers to a
group of command-line switches grouped into one L</switch cluster>.)

=item byte

A piece of data worth eight L<bits|/bit> in most places.

=item bytecode

A pidgin-like language spoken among 'droids when they don't wish to
reveal their orientation (see L</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
non-architecture-dependent sequence of bytes.

=back

=head2 C

=over 4

=item C

A language beloved by many for its inside-out L</type> definitions,
inscrutable L</precedence> rules, and heavy L</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 C preprocessor

The 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 call by reference

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

=item call by value

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

=item callback

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

=item canonical

Reduced to a standard form to facilitate comparison.

=item capturing

The use of parentheses around a L</subpattern> in a L</regular
expression> to store the matched L</substring> as a L</backreference>.
(Captured strings are also returned as a list in L</list context>.)

=item character

A small integer representative of a unit of orthography.
Historically, characters were usually stored as fixed-width integers
(typically in a byte, or maybe two, depending on the character set),
but with the advent of UTF-8, characters are often stored in a
variable number of bytes depending on the size of the integer that
represents the character.  Perl manages this transparently for you,
for the most part.

=item character class

A square-bracketed list of characters used in a L</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 predefined L</character class> matchable by the C<\p>
L</metasymbol>.  Many standard properties are defined for L</Unicode>.

=item circumfix operator

An L</operator> that surrounds its L</operand>, like the angle
operator, or parentheses, or a hug.

=item class

A user-defined L</type>, implemented in Perl via a L</package> that
provides (either directly or by inheritance) L<methods|/method> (that
is, L<subroutines|/subroutine>) to handle L<instances|/instance> of
the class (its L<objects|/object>).  See also L</inheritance>.

=item class method

A L</method> whose L</invocant> is a L</package> name, not an
L</object> reference.  A method associated with the class as a whole.

=item client

In networking, a L</process> that initiates contact with a L</server>
process in order to exchange data and perhaps receive a service.

=item cloister

A L</cluster> used to restrict the scope of a L</regular expression
modifier>.

=item closure

An L</anonymous> subroutine that, when a reference to it is generated
at run time, keeps track of the identities of externally visible
L<lexical variables|/lexical variable> even after those lexical
variables have supposedly gone out of L</scope>.  They're called
"closures" because this sort of behavior gives mathematicians a sense
of closure.

=item cluster

A parenthesized L</subpattern> used to group parts of a L</regular
expression> into a single L</atom>.

=item CODE

The word returned by the L<ref|perlfunc/ref> function when you apply
it to a reference to a subroutine.  See also L</CV>.

=item code generator

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

=item code subpattern

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

=item collating sequence

The order into which L<characters|/character> sort.  This is used by
L</string> comparison routines to decide, for example, where in this
glossary to put "collating sequence".

=item command

In L</shell> 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 L</statement>, which might start with a L</label> and
typically ends with a semicolon.

=item command buffering

A mechanism in Perl that lets you store up the output of each Perl
L</command> and then flush it out as a single request to the
L</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 L</file> or L</pipe> is to use
L</block buffering>.

=item command name

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

=item command-line arguments

The L<values|/value> you supply along with a program name when you
tell a L</shell> to execute a L</command>.  These values are passed to
a Perl program through C<@ARGV>.

=item comment

A 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 L</file> (or L</string>, in the case of L<eval|perlfunc/eval>)
that is currently being compiled.

=item compile phase

Any time before Perl starts running your main program.  See also
L</run phase>.  Compile phase is mostly spent in L</compile time>, but
may also be spent in L</run time> when C<BEGIN> blocks,
L<use|perlfunc/use> declarations, or constant subexpressions are being
evaluated.  The startup and import code of any L<use|perlfunc/use>
declaration is also run during compile phase.

=item compile time

The 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 L</run time>.

=item compiler

Strictly 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 (L<syntax trees|/syntax tree>) within the I<perl>
process itself, which the L</interpreter> then interprets.  There are,
however, extension L<modules|/module> to get Perl to act more like a
"real" compiler.  See L<O>.

=item composer

A "constructor" for a L</referent> that isn't really an L</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 L<perlref/Making
References>.

=item concatenation

The process of gluing one cat's nose to another cat's tail.  Also, a
similar operation on two L<strings|/string>.

=item conditional

Something "iffy".  See L</Boolean context>.

=item connection

In telephony, the temporary electrical circuit between the caller's
and the callee's phone.  In networking, the same kind of temporary
circuit between a L</client> and a L</server>.

=item construct

As a noun, a piece of syntax made up of smaller pieces.  As a
transitive verb, to create an L</object> using a L</constructor>.

=item constructor

Any L</class method>, instance L</method>, or L</subroutine>
that composes, initializes, blesses, and returns an L</object>.
Sometimes we use the term loosely to mean a L</composer>.

=item context

The surroundings, or environment.  The context given by the
surrounding code determines what kind of data a particular
L</expression> is expected to return.  The three primary contexts are
L</list context>, L</scalar context>, and L</void context>.  Scalar
context is sometimes subdivided into L</Boolean context>, L</numeric
context>, L</string context>, and L</void context>.  There's also a
"don't care" scalar context (which is dealt with in Programming Perl,
Third Edition, Chapter 2, "Bits and Pieces" if you care).

=item continuation

The treatment of more than one physical L</line> as a single logical
line.  L</Makefile> lines are continued by putting a backslash before
the L</newline>.  Mail headers as defined by 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 L</whitespace>
(including newlines) is gleefully ignored.  Usually.

=item core dump

The corpse of a L</process>, in the form of a file left in the
L</working directory> of the process, usually as a result of certain
kinds of fatal error.

=item CPAN

The Comprehensive Perl Archive Network.  (See L<perlfaq2/What modules and extensions are available for Perl?  What is CPAN?  What does CPANE<sol>srcE<sol>... mean?>).

=item cracker

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

=item current package

The L</package> in which the current statement is compiled.  Scan
backwards in the text of your program through the current L<lexical
scope|/lexical scoping> or any enclosing lexical scopes till you find
a package declaration.  That's your current package name.

=item current working directory

See L</working directory>.

=item currently selected output channel

The last L</filehandle> that was designated with
L<select|perlfunc/select>(C<FILEHANDLE>); L</STDOUT>, if no filehandle
has been selected.

=item CV

An internal "code value" typedef, holding a L</subroutine>.  The L</CV>
type is a subclass of L</SV>.

=back

=head2 D

=over 4

=item dangling statement

A bare, single L</statement>, without any braces, hanging off an C<if>
or C<while> conditional.  C allows them.  Perl doesn't.

=item data structure

How your 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-shaped tree.

=item data type

A set of 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 and 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 L</concatenation>.  Compound types made of a
number of smaller pieces generally have operations to compose and
decompose them, and perhaps to rearrange them.  L<Objects|/object>
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()>
L</method>.

=item datagram

A packet of data, such as a L</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 L</IP> level, but
L</stream> protocols such as L</TCP> hide this from your program.)

=item DBM

Stands for "Data Base Management" routines, a set of routines that
emulate an L</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
L</hash> across multiple invocations.  You can L<tie|perlfunc/tie>
your hash variables to various DBM implementations--see L<AnyDBM_File>
and L<DB_File>.

=item declaration

An L</assertion> 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
L</statement> for its opposite.  Note that some declarations also
function as statements.  Subroutine declarations also act as
definitions if a body is supplied.

=item decrement

To 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 L</value> chosen for you if you don't supply a value of your own.

=item defined

Having a 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 L</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
L</false> and L<perlfunc/defined>.

=item delimiter

A L</character> or L</string> that sets bounds to an arbitrarily-sized
textual object, not to be confused with a L</separator> or
L</terminator>.  "To delimit" really just means "to surround" or "to
enclose" (like these parentheses are doing).

=item dereference

A fancy computer science term meaning "to follow a L</reference> to
what it points to".  The "de" part of it refers to the fact that
you're taking away one level of L</indirection>.

=item derived class

A L</class> that defines some of its L<methods|/method> in terms of a
more generic class, called a L</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 L</file descriptor>.

=item destroy

To deallocate the memory of a L</referent> (first triggering its
C<DESTROY> method, if it has one).

=item destructor

A special L</method> that is called when an L</object> is thinking
about L<destroying|/destroy> itself.  A Perl program's C<DESTROY>
method doesn't do the actual destruction; Perl just
L<triggers|/trigger> the method in case the L</class> wants to do any
associated cleanup.

=item device

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

=item directive

A L</pod> directive.  See L<perlpod>.

=item directory

A special file that contains other files.  Some L<operating
systems|/operating system> call these "folders", "drawers", or
"catalogs".

=item directory handle

A name that represents a particular instance of opening a directory to
read it, until you close it.  See the L<opendir|perlfunc/opendir>
function.

=item dispatch

To send 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 L<references|/reference> or, in the case of object
L<methods|/method>, by traversing the inheritance tree looking for the
most specific definition for the method.

=item distribution

A standard, 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 dweomer

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

=item dwimmer

DWIM is an acronym for "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 L</dweomer> instead.

=item dynamic scoping

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

=back

=head2 E

=over 4

=item eclectic

Derived from many sources.  Some would say I<too> many.

=item element

A basic building block.  When you're talking about an L</array>, it's
one of the items that make up the array.

=item embedding

When 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 that an empty L</derived class> should behave exactly like
its L</base class>.

=item en passant

When you change a L</value> as it is being copied.  [From French, "in
passing", as in the exotic pawn-capturing maneuver in chess.]

=item encapsulation

The veil of abstraction separating the L</interface> from the
L</implementation> (whether enforced or not), which mandates that all
access to an L</object>'s state be through L<methods|/method> alone.

=item endian

See L</little-endian> and L</big-endian>.

=item environment

The collective set of L<environment variables|/environment variable>
your L</process> inherits from its parent.  Accessed via C<%ENV>.

=item environment variable

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

=item EOF

End of File.  Sometimes used metaphorically as the terminating string
of a L</here document>.

=item errno

The error number returned by a L</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 L</exception> or L</fatal error>.

=item escape sequence

See L</metasymbol>.

=item exception

A fancy term for an error.  See L</fatal error>.

=item exception handling

The way a program responds to an error.  The exception handling
mechanism in Perl is the L<eval|perlfunc/eval> operator.

=item exec

To throw away the current L</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 L</file> that is specially marked to tell the L</operating system>
that it's okay to run this file as a program.  Usually shortened to
"executable".

=item execute

To run a L<program|/executable file> or L</subroutine>.  (Has nothing
to do with the L<kill|perlfunc/kill> built-in, unless you're trying to
run a L</signal handler>.)

=item execute bit

The 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 L</status>.

=item export

To make symbols from a L</module> available for L</import> by other modules.

=item expression

Anything you can legally say in a spot where a L</value> is required.
Typically composed of L<literals|/literal>, L<variables|/variable>,
L<operators|/operator>, L<functions|/function>, and L</subroutine>
calls, not necessarily in that order.

=item extension

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

=back

=head2 F

=over 4

=item false

In Perl, any value 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 Question (although not necessarily frequently
answered, especially if the answer appears in the Perl FAQ shipped
standard with Perl).

=item fatal error

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

=item field

A single piece of numeric or string data that is part of a longer
L</string>, L</record>, or L</line>.  Variable-width fields are usually
split up by L<separators|/separator> (so use L<split|perlfunc/split> to
extract the fields), while fixed-width fields are usually at fixed
positions (so use L<unpack|perlfunc/unpack>).  L<Instance
variables|/instance variable> are also known as fields.

=item FIFO

First In, First Out.  See also L</LIFO>.  Also, a nickname for a
L</named pipe>.

=item file

A named collection of data, usually stored on disk in a L</directory>
in a L</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 number the L</operating system> uses to keep track of which
opened L</file> you're talking about.  Perl hides the file descriptor
inside a L</standard IE<sol>O> stream and then attaches the stream to
a L</filehandle>.

=item file test operator

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

=item fileglob

A "wildcard" match on L<filenames|/filename>.  See the
L<glob|perlfunc/glob> function.

=item filehandle

An identifier (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 file.  This name is listed in a L</directory>, and you
can use it in an L<open|perlfunc/open> to tell the L</operating
system> exactly which file you want to open, and associate the file
with a L</filehandle> which will carry the subsequent identity of that
file in your program, until you close it.

=item filesystem

A set of L<directories|/directory> and L<files|/file> 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 filter

A program designed to take a L</stream> of input and transform it into
a stream of output.

=item flag

We tend to avoid this term because it means so many things.  It may
mean a command-line L</switch> that takes no argument
itself (such as Perl's B<-n> and B<-p>
flags) or, less frequently, a single-bit indicator (such as the
C<O_CREAT> and C<O_EXCL> flags used in
L<sysopen|perlfunc/sysopen>).

=item floating point

A 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 L<integers|/integer>.  Floating-point numbers are mere
approximations of real numbers.

=item flush

The act of emptying a L</buffer>, often before it's full.

=item FMTEYEWTK

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

=item fork

To create a child L</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 generic names by which a L</subroutine> knows its
L<arguments|/argument>.  In many languages, formal arguments are
always given individual names, but 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
L<my|perlfunc/my> list.  See also L</actual arguments>.

=item format

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

=item function

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

=item funny character

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

=item garbage collection

A misnamed feature--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.)

=back

=head2 G

=over 4

=item GID

Group ID--in Unix, the numeric group ID that the L</operating system>
uses to identify you and members of your L</group>.

=item glob

Strictly, the 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 L</fileglob> and L</typeglob>.

=item global

Something you can see from anywhere, usually used of
L<variables|/variable> and L<subroutines|/subroutine> 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 L</package>.  Global variables can be declared with
L<our|perlfunc/our>.  See L<perlfunc/our>.

=item global destruction

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

=item glue language

A 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 pieces you're dealing with, mentally speaking.

=item greedy

A L</subpattern> whose L</quantifier> wants to match as many things as
possible.

=item grep

Originally 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
L<grep|perlfunc/grep> function that searches a list for elements
matching any given criterion, whereas the I<grep>(1) program searches
for lines matching a L</regular expression> in one or more files.

=item group

A set of users 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" typedef, holding a L</typeglob>.  The L</GV>
type is a subclass of L</SV>.

=back

=head2 H

=over 4

=item hacker

Someone 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 L<crackers|/cracker> or clueless L<script
kiddies|/script kiddie>.  If you confuse them, we will presume that
you are either evil or clueless.

=item handler

A L</subroutine> or L</method> that is called by Perl when your
program needs to respond to some internal event, such as a L</signal>,
or an encounter with an operator subject to L</operator overloading>.
See also L</callback>.

=item hard reference

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

=item hash

An unordered association of L</key>/L</value> pairs, stored such that
you can easily use a string L</key> to look up its associated data
L</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 structure used internally by Perl for implementing associative
arrays (hashes) efficiently.  See also L</bucket>.

=item header file

A file 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 L<perlfunc/require>.
(Header files have been superseded by the L</module> mechanism.)

=item here document

So called because of a similar construct in L<shells|/shell> that
pretends that the L<lines|/line> following the L</command> are a
separate L</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 number in base 16, "hex" for short.  The digits for 10 through 16
are customarily represented by the letters C<a> through C<f>.
Hexadecimal constants in Perl start with C<0x>.  See also
L<perlfunc/hex>.

=item home directory

The 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<(getpwuid($E<lt>))[7]>.
(Some platforms do not have a concept of a home directory.)

=item host

The computer on which a program or other data resides.

=item hubris

Excessive pride, the sort of thing Zeus zaps you for.  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 L</laziness> and L</impatience>.

=item HV

Short for a "hash value" typedef, which holds Perl's internal
representation of a hash.  The L</HV> type is a subclass of L</SV>.

=back

=head2 I

=over 4

=item identifier

A legally formed name for most anything in which a computer program
might be interested.  Many languages (including Perl) allow
identifiers that start with a letter and contain letters and digits.
Perl also counts the underscore character as a valid letter.  (Perl
also has more complicated names, such as L</qualified> names.)

=item impatience

The anger 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 L</laziness> and L</hubris>.

=item implementation

How a 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 L</interface>.

=item import

To gain access to symbols that are exported from another module.  See
L<perlfunc/use>.

=item increment

To increase the value of something by 1 (or by some other number, if
so specified).

=item indexing

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

=item indirect filehandle

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

=item indirect object

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

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

=item indirect object slot

The syntactic 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.) L</STDERR> is in the indirect object slot here:

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

=item indirection

If something in a program isn't the value you're looking for but
indicates where the value is, that's indirection.  This can be done
with either L<symbolic references|/symbolic reference> or L<hard
references|/hard reference>.

=item infix

An L</operator> that comes in between its L<operands|/operand>, such
as multiplication in C<24 * 7>.

=item inheritance

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

=item instance

Short for "an instance of a class", meaning an L</object> of that L</class>.

=item instance variable

An L</attribute> of an L</object>; data stored with the particular
object rather than with the class as a whole.

=item integer

A number 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 a piece of code promises to provide forever, in contrast to
its L</implementation>, which it should feel free to change whenever it
likes.

=item interpolation

The insertion of 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
L</LIST>.

=item interpreter

Strictly speaking, 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 L<compilers|/compiler> 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 (L<syntax trees|/syntax tree>) within the I<perl> process itself,
which the Perl L</run time> system then interprets.

=item invocant

The agent on whose behalf a L</method> is invoked.  In a L</class>
method, the invocant is a package name.  In an L</instance> method,
the invocant is an object reference.

=item invocation

The act of calling up a deity, daemon, program, method, subroutine, or
function to get it 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 output to, a L</file> or L</device>.

=item IO

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

=item IP

Internet Protocol, or Intellectual Property.

=item IPC

Interprocess Communication.

=item is-a

A relationship between two L<objects|/object> 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
L</base class> and a specific L</derived class>.  Oddly enough,
Platonic classes don't always have Platonic relationships--see
L</inheritance>.

=item iteration

Doing something repeatedly.

=item iterator

A special 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
L<each|perlfunc/each> through it.

=item IV

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

=back

=head2 J

=over 4

=item JAPH

"Just Another Perl Hacker," a 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 ungoing Obfuscated Perl
Contest seen in Usenix signatures.

=back

=head2 K

=over 4

=item key

The string index to a L</hash>, used to look up the L</value>
associated with that key.

=item keyword

See L</reserved words>.

=back

=head2 L

=over 4

=item label

A name you give to a L</statement> so that you can talk about that
statement elsewhere in the program.

=item laziness

The 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 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 L</impatience> and
L</hubris>.

=item left shift

A L</bit shift> that multiplies the number by some power of 2.

=item leftmost longest

The preference of the L</regular expression> engine to match the
leftmost occurrence of a L</pattern>, then given a position at which a
match will occur, the preference for the longest match (presuming the
use of a L</greedy> quantifier).  See L<perlre> for I<much> more on
this subject.

=item lexeme

Fancy term for a L</token>.

=item lexer

Fancy term for a L</tokener>.

=item lexical analysis

Fancy term for L</tokenizing>.

=item lexical scoping

Looking at your I<Oxford English Dictionary> through a microscope.
(Also known as L</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 the lexical scope in
which they are declared.  --Syn. L</static scoping>.
--Ant. L</dynamic scoping>.

=item lexical variable

A L</variable> subject to L</lexical scoping>, declared by
L<my|perlfunc/my>.  Often just called a "lexical".  (The
L<our|perlfunc/our> declaration declares a lexically scoped name for a
global variable, which is not itself a lexical variable.)

=item library

Generally, a 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 L<modules|/module> on your
system.

=item LIFO

Last In, First Out.  See also L</FIFO>.  A LIFO is usually called a
L</stack>.

=item line

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

=item line buffering

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

=item line number

The 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 noun, a name in a L</directory>, representing a L</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 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 construct representing a comma-separated list of
expressions, evaluated to produce a L</list value>.  Each
L</expression> in a L</LIST> is evaluated in L</list context> and
interpolated into the list value.

=item list

An ordered set of scalar values.

=item list context

The situation in which an L</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 L</LIST> of arguments tell
those arguments that they should produce a list value.  See also
L</context>.

=item list operator

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

=item list value

An unnamed 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 L</list context>.

=item literal

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

=item little-endian

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

=item local

Not meaning the same thing everywhere.  A global variable in Perl can
be localized inside a L<dynamic scope|/dynamic scoping> via the
L<local|perlfunc/local> operator.

=item logical operator

Symbols representing the concepts "and", "or", "xor", and "not".

=item lookahead

An L</assertion> that peeks at the string to the right of the current
match location.

=item lookbehind

An L</assertion> that peeks at the string to the left of the current
match location.

=item loop

A construct that performs something repeatedly, like a roller coaster.

=item loop control statement

Any statement within the body of a loop that can make a loop
prematurely stop looping or skip an L</iteration>.  Generally you
shouldn't try this on roller coasters.

=item loop label

A kind 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 lvaluable

Able to serve as an L</lvalue>.

=item lvalue

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

=item lvalue modifier

An adjectival pseudofunction that warps the meaning of an L</lvalue>
in some declarative fashion.  Currently there are three lvalue
modifiers: L<my|perlfunc/my>, L<our|perlfunc/our>, and
L<local|perlfunc/local>.

=back

=head2 M

=over 4

=item magic

Technically 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 L</increment> operator that knows how to bump up alphabetics as
well as numbers.

=item magical variables

Special variables 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 controls the compilation of a program.  Perl programs
don't usually need a L</Makefile> because the Perl compiler has plenty
of self-control.

=item man

The Unix program that displays online documentation (manual pages) for
you.

=item manpage

A "page" from the 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 L<commands|/command>, L<syscalls|/syscall>,
L</library> L<functions|/function>, L<devices|/device>,
L<protocols|/protocol>, L<files|/file>, and such.  In this book, we
call any piece of standard Perl documentation (like I<perlop> or
I<perldelta>) a manpage, no matter what format it's installed in on
your system.

=item matching

See L</pattern matching>.

=item member data

See L</instance variable>.

=item memory

This always means your main memory, not your disk.  Clouding the issue
is the fact that your machine may implement L</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 L</character> that is I<not> supposed to be treated normally.  Which
characters are to be treated specially as metacharacters varies
greatly from context to context.  Your L</shell> will have certain
metacharacters, double-quoted Perl L<strings|/string> have other
metacharacters, and L</regular expression> patterns have all the
double-quote metacharacters plus some extra ones of their own.

=item metasymbol

Something we'd call a L</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 action that an L</object> can take if you tell it to.  See
L<perlobj>.

=item minimalism

The belief 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 context of the L<stat> syscall, refers to the field holding
the L</permission bits> and the type of the L</file>.

=item modifier

See L</statement modifier>, L</regular expression modifier>, and
L</lvalue modifier>, not necessarily in that order.

=item module

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

=item modulus

An integer divisor when you're interested in the remainder instead of
the quotient.

=item monger

Short for Perl Monger, a purveyor of Perl.

=item mortal

A temporary value scheduled to die when the current statement
finishes.

=item multidimensional array

An array with multiple subscripts for finding a single element.  Perl
implements these using L<references|/reference>--see L<perllol> and
L<perldsc>.

=item multiple inheritance

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

=back

=head2 N

=over 4

=item named pipe

A L</pipe> with a name embedded in the L</filesystem> so that it can
be accessed by two unrelated L<processes|/process>.

=item namespace

A domain of names.  You needn't worry about whether the names in one
such domain have been used in another.  See L</package>.

=item network address

The most important attribute of a socket, like your telephone's
telephone number.  Typically an IP address.  See also L</port>.

=item newline

A single 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, which allows you to mount a remote filesystem as
if it were local.

=item null character

A character with the ASCII value of zero.  It's used by C to terminate
strings, but Perl allows strings to contain a null.

=item null list

A L</list value> with zero elements, represented in Perl by C<()>.

=item null string

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

=item numeric context

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

=item NV

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

=item nybble

Half a L</byte>, equivalent to one L</hexadecimal> digit, and worth
four L<bits|/bit>.

=back

=head2 O

=over 4

=item object

An L</instance> of a L</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 in base 8.  Only the digits 0 through 7 are allowed.  Octal
constants in Perl start with 0, as in 013.  See also the
L<oct|perlfunc/oct> function.

=item offset

How many 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 computer program crammed into one line of text.

=item open source software

Programs 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 L</expression> that yields a L</value> that an L</operator>
operates on.  See also L</precedence>.

=item operating system

A special program that runs on the bare machine and hides the gory
details of managing L<processes|/process> and L<devices|/device>.
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
L</architecture> and L</platform>.

=item operator

A gizmo 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
L<types|/type> of data you give as its arguments
(L<operands|/operand>) and what type of data you want back from it.

=item operator overloading

A kind of L</overloading> that you can do on built-in
L<operators|/operator> to make them work on L<objects|/object> as if
the objects were ordinary scalar values, but with the actual semantics
supplied by the object class.  This is set up with the L<overload>
L</pragma>.

=item options

See either L<switches|/switch> or L</regular expression modifier>.

=item overloading

Giving 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 L</context>.

=item overriding

Hiding or invalidating some other definition of the same name.  (Not
to be confused with L</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 L</subroutine> to hide a built-in L</function> of the
same name (see L<perlsub/Overriding Built-in Functions>) and to
describe how you can define a replacement L</method> in a L</derived
class> to hide a L</base class>'s method of the same name (see
L<perlobj>).

=item owner

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

=back

=head2 P

=over 4

=item package

A L</namespace> for global L<variables|/variable>,
L<subroutines|/subroutine>, and the like, such that they can be kept
separate from like-named L<symbols|/symbol> 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 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 for L</scratchpad>.

=item parameter

See L</argument>.

=item parent class

See L</base class>.

=item parse tree

See L</syntax tree>.

=item parsing

The subtle but sometimes brutal art of attempting to turn your
possibly malformed program into a valid L</syntax tree>.

=item patch

To 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 I<patch>(1) program when you want to fix a bug or
upgrade your old version.

=item PATH

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

=item pathname

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

=item pattern

A template used in L</pattern matching>.

=item pattern matching

Taking a pattern, usually a L</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 permission bits

Bits that the L</owner> of a file sets or unsets to allow or disallow
access to other people.  These flag bits are part of the L</mode> word
returned by the L<stat|perlfunc/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 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 direct L</connection> that carries the output of one L</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 series of L<processes|/process> all in a row, linked by
L<pipes|/pipe>, where each passes its output stream to the next.

=item platform

The 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: machine, operating system, libraries, compiler, or
system configuration.  The I<perl> interpreter has to be 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 markup used to embed documentation into your Perl code.  See
L<perlpod>.

=item pointer

A L</variable> 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 L<keys|/key> and L</variable> names, or L<hard
references|/hard reference>, which aren't pointers (but act like
pointers and do in fact contain pointers).

=item polymorphism

The notion that you can tell an L</object> to do something generic,
and the object will interpret the command in different ways depending
on its type.  [E<lt>Gk many shapes]

=item port

The 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 upon a time, C code compilable under both BSD and SysV.  In
general, code that can be easily converted to run on another
L</platform>, where "easily" can be defined however you like, and
usually is.  Anything may be considered portable if you try hard
enough.  See I<mobile home> or I<London Bridge>.

=item porter

Someone who "carries" software from one L</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 POSIX

The Portable Operating System Interface specification.

=item postfix

An L</operator> that follows its L</operand>, as in C<$x++>.

=item pp

An internal shorthand for a "push-pop" code, that is, C code
implementing Perl's stack machine.

=item pragma

A standard module whose practical hints and suggestions are received
(and possibly ignored) at compile time.  Pragmas are named in all
lowercase.

=item precedence

The 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 L</operator> that precedes its L</operand>, as in C<++$x>.

=item preprocessing

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

=item procedure

A L</subroutine>.

=item process

An 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 L<fork|perlfunc/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 generator

A system that algorithmically writes code for you in a high-level
language.  See also L</code generator>.

=item progressive matching

L<Pattern matching|/pattern matching> that picks up where it left off before.

=item property

See either L</instance variable> or L</character property>.

=item protocol

In networking, an agreed-upon way of sending messages back and forth
so that neither correspondent will get too confused.

=item prototype

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

=item pseudofunction

A construct that sometimes looks like a function but really isn't.
Usually reserved for L</lvalue> modifiers like L<my|perlfunc/my>, for
L</context> modifiers like L<scalar|perlfunc/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

A reference to an array whose initial element happens to hold a
reference to a hash.  You can treat a pseudohash reference as either
an array reference or a hash reference.

=item pseudoliteral

An L</operator> that looks something like a L</literal>, such as the
output-grabbing operator, C<`>I<C<command>>C<`>.

=item public domain

Something not owned by anybody.  Perl is copyrighted and is thus
I<not> in the public domain--it's just L</freely available> and
L</freely redistributable>.

=item pumpkin

A notional "baton" handed around the Perl community indicating who is
the lead integrator in some arena of development.

=item pumpking

A L</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 "pointer value", which is Perl Internals Talk for a C<char*>.

=back

=head2 Q

=over 4

=item qualified

Possessing a 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 component of a L</regular expression> specifying how many times the
foregoing L</atom> may occur.

=back

=head2 R

=over 4

=item readable

With 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 rites performed by a parent L</process> on behalf of a
deceased child process so that it doesn't remain a L</zombie>.  See
the L<wait|perlfunc/wait> and L<waitpid|perlfunc/waitpid> function
calls.

=item record

A set of related data values in a L</file> or L</stream>, often
associated with a unique L</key> field.  In Unix, often commensurate
with a L</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 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 look to find a pointer to information somewhere else.  (See
L</indirection>.)  References come in two flavors, L<symbolic
references|/symbolic reference> and L<hard references|/hard
reference>.

=item referent

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

=item regex

See L</regular expression>.

=item regular expression

A single 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
L<perlre>.

=item regular expression modifier

An option on a pattern or substitution, such as C</i> to render the
pattern case insensitive.  See also L</cloister>.

=item regular file

A L</file> that's not a L</directory>, a L</device>, a named L</pipe>
or L</socket>, or a L</symbolic link>.  Perl uses the C<-f> file test
operator to identify regular files.  Sometimes called a "plain" file.

=item relational operator

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

=item reserved words

A word with a specific, built-in meaning to a L</compiler>, such as
C<if> or L<delete|perlfunc/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 L<labels|/label> or L<filehandles|/filehandle>.  Also called
"keywords".

=item return value

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

=item RFC

Request For Comment, which despite the timid connotations is the name
of a series of important standards documents.

=item right shift

A L</bit shift> that divides a number by some power of 2.

=item root

The superuser (UID == 0).  Also, the top-level directory of the
filesystem.

=item RTFM

What you are told when someone thinks you should Read The Fine Manual.

=item run phase

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

=item run time

The time 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 L</compile
time>.

=item run-time pattern

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

=item RV

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

=item rvalue

A L</value> that you might find on the right side of an
L</assignment>.  See also L</lvalue>.

=back

=head2 S

=over 4

=item scalar

A simple, singular value; a number, L</string>, or L</reference>.

=item scalar context

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

=item scalar literal

A number or quoted L</string>--an actual L</value> in the text of your
program, as opposed to a L</variable>.

=item scalar value

A value that happens to be a L</scalar> as opposed to a L</list>.

=item scalar variable

A L</variable> prefixed with C<$> that holds a single value.

=item scope

How far away you can see a variable from, looking through one.  Perl
has two visibility mechanisms: it does L</dynamic scoping> of
L<local|perlfunc/local> L<variables|/variable>, meaning that the rest
of the L</block>, and any L<subroutines|/subroutine> that are called
by the rest of the block, can see the variables that are local to the
block.  Perl does L</lexical scoping> of L<my|perlfunc/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 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 text L</file> that is a program intended to be L<executed|/execute>
directly rather than L<compiled|/compiler> to another form of file
before execution.  Also, in the context of L</Unicode>, a writing
system for a particular language or group of languages, such as Greek,
Bengali, or Klingon.

=item script kiddie

A L</cracker> who is not a L</hacker>, but knows just enough to run
canned scripts.  A cargo-cult programmer.

=item sed

A venerable Stream EDitor from which Perl derives some of its ideas.

=item semaphore

A fancy kind of interlock that prevents multiple L<threads|/thread> or
L<processes|/process> from using up the same resources simultaneously.

=item separator

A L</character> or L</string> that keeps two surrounding strings from
being confused with each other.  The L<split|perlfunc/split> function
works on separators.  Not to be confused with L<delimiters|/delimiter>
or L<terminators|/terminator>.  The "or" in the previous sentence
separated the two alternatives.

=item serialization

Putting a fancy L</data structure> into linear order so that it can be
stored as a L</string> in a disk file or database or sent through a
L</pipe>.  Also called marshalling.

=item server

In networking, a L</process> that either advertises a L</service> or
just hangs around at a known location and waits for L<clients|/client>
who need service to get in touch with it.

=item service

Something 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 the L<getservent|perlfunc/getservent> function.

=item setgid

Same as L</setuid>, only having to do with giving away L</group>
privileges.

=item setuid

Said of a program that runs with the privileges of its L</owner>
rather than (as is usually the case) the privileges of whoever is
running it.  Also describes the bit in the mode word (L</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 L</memory> accessible by two different
L<processes|/process> who otherwise would not see each other's memory.

=item shebang

Irish for the 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 L</command>-line L</interpreter>.  The program that interactively
gives you a prompt, accepts one or more L<lines|/line> of input, and
executes the programs you mentioned, feeding each of them their proper
L<arguments|/argument> 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 that happens when you evaluate an L</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 L<write|perlfunc/write> or
L<print|perlfunc/print> on the currently selected filehandle.

=item signal

A bolt out of the blue; that is, an event triggered by the
L</operating system>, probably when you're least expecting it.

=item signal handler

A L</subroutine> that, 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 L</execute>.  Under Perl, bolts out of the blue are
called signals, and you send them with the L<kill|perlfunc/kill>
built-in.  See L<perlvar/%SIG> and L<perlipc/Signals>.

=item single inheritance

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

=item slice

A selection of any number of L<elements|/element> from a L</list>,
L</array>, or L</hash>.

=item slurp

To read an entire L</file> into a L</string> in one operation.

=item socket

An endpoint for network communication among multiple
L<processes|/process> that works much like a telephone or a post
office box.  The most important thing about a socket is its L</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

See L</symbolic reference>.

=item source filter

A special kind of L</module> that does L</preprocessing> on your
script just before it gets to the L</tokener>.

=item stack

A 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 L</LIFO>.

=item standard

Included in the official Perl distribution, as in a standard module, a
standard tool, or a standard Perl L</manpage>.

=item standard error

The default output L</stream> for nasty remarks that don't belong in
L</standard output>.  Represented within a Perl program by the
L</filehandle> L</STDERR>.  You can use this stream explicitly, but the
L<die|perlfunc/die> and L<warn|perlfunc/warn> built-ins write to your
standard error stream automatically.

=item standard I/O

A standard C library for doing L<buffered|/buffer> input and output to
the L</operating system>.  (The "standard" of standard I/O is only
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
L</flush> the buffer more often, just set the C<$|> variable to a true
value.

=item standard input

The default input L</stream> for your program, which if possible
shouldn't care where its data is coming from.  Represented within a
Perl program by the L</filehandle> L</STDIN>.

=item standard output

The default output L</stream> for your program, which if possible
shouldn't care where its data is going.  Represented within a Perl
program by the L</filehandle> L</STDOUT>.

=item stat structure

A special internal spot in which Perl keeps the information about the
last L</file> on which you requested information.

=item statement

A L</command> to 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 L</declaration>, which doesn't tell the computer
to do anything, but just to learn something.

=item statement modifier

A L</conditional> or L</loop> that you put after the L</statement>
instead of before, if you know what we mean.

=item static

Varying 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
L</variable>, L</subroutine>, or L</method>.  In Perl culture, the
word is politely avoided.

=item static method

No such thing.  See L</class method>.

=item static scoping

No such thing.  See L</lexical scoping>.

=item static variable

No such thing.  Just use a L</lexical variable> in a scope larger than
your L</subroutine>.

=item status

The L</value> returned to the parent L</process> when one of its child
processes dies.  This value is placed in the special variable C<$?>.
Its upper eight L<bits|/bit> 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 L<perlfunc/system>.

=item STDERR

See L</standard error>.

=item STDIN

See L</standard input>.

=item STDIO

See L</standard IE<sol>O>.

=item STDOUT

See L</standard output>.

=item stream

A flow 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 L</interface>--the underlying L</implementation> may
well break your data up into separate packets for delivery, but this
is hidden from you.

=item string

A sequence of characters such as "He said !@#*&%@#*?!".  A string does
not have to be entirely printable.

=item string context

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

=item stringification

The process of producing a L</string> representation of an abstract
object.

=item struct

C keyword introducing a structure definition or name.

=item structure

See L</data structure>.

=item subclass

See L</derived class>.

=item subpattern

A component of a L</regular expression> pattern.

=item subroutine

A named or otherwise accessible piece of program that can be invoked
from elsewhere in the program in order to accomplish some sub-goal of
the program.  A subroutine is often parameterized to accomplish
different but related things depending on its input
L<arguments|/argument>.  If the subroutine returns a meaningful
L</value>, it is also called a L</function>.

=item subscript

A L</value> that indicates the position of a particular L</array>
L</element> in an array.

=item substitution

Changing parts of a string via the C<s///> operator.  (We avoid use of
this term to mean L</variable interpolation>.)

=item substring

A portion of a L</string>, starting at a certain L</character>
position (L</offset>) and proceeding for a certain number of
characters.

=item superclass

See L</base class>.

=item superuser

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

=item SV

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

=item switch

An 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 L</switch statement>.

=item switch cluster

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

=item switch statement

A program technique that lets you evaluate an L</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<for>.  See L<perlsyn/Basic BLOCKs and
Switch Statements>.

=item symbol

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

=item symbol table

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

=item symbolic debugger

A program that lets you step through the L<execution|/execute> 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 filename that points to the real L</filename>, which in
turn points to the real L</file>.  Whenever the L</operating system>
is trying to parse a L</pathname> containing a symbolic link, it
merely substitutes the new name and continues parsing.

=item symbolic reference

A variable whose value is the name of another variable or subroutine.
By L<dereferencing|/dereference> the first variable, you can get at
the second one.  Symbolic references are illegal under L<use strict
'refs'|strict/strict refs>.

=item synchronous

Programming 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 alternative way of writing something more easily; a shortcut.

=item syntax

From Greek, "with-arrangement".  How things (particularly symbols) are
put together with each other.

=item syntax tree

An internal representation of your program wherein lower-level
L<constructs|/construct> dangle off the higher-level constructs
enclosing them.

=item syscall

A L</function> call directly to the L</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 L<system|perlfunc/system> function, which
actually involves many syscalls.  To avoid any confusion, we nearly
always use say "syscall" for something you could call indirectly via
Perl's L<syscall|perlfunc/syscall> function, and never for something
you would call with Perl's L<system|perlfunc/system> function.

=back

=head2 T

=over 4

=item tainted

Said of 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
L</setuid> (or L</setgid>) program, or if you use the B<-T> switch.

=item TCP

Short for 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
L</stream> of bytes.  (Usually.)

=item term

Short for a "terminal", that is, a leaf node of a L</syntax tree>.  A
thing that functions grammatically as an L</operand> for the operators
in an expression.

=item terminator

A L</character> or L</string> that marks the end of another string.
The C<$/> variable contains the string that terminates a
L<readline|perlfunc/readline> operation, which L<chomp|perlfunc/chomp>
deletes from the end.  Not to be confused with
L<delimiters|/delimiter> or L<separators|/separator>.  The period at
the end of this sentence is a terminator.

=item ternary

An L</operator> taking three L<operands|/operand>.  Sometimes
pronounced L</trinary>.

=item text

A L</string> or L</file> containing primarily printable characters.

=item thread

Like a forked process, but without L</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 each other.  See L<threads>.

=item tie

The bond between a magical variable and its implementation class.  See
L<perlfunc/tie> and L<perltie>.

=item TMTOWTDI

There's More Than One Way To Do It, the Perl Motto.  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.)  Pronounced TimToady.

=item token

A morpheme in a programming language, the smallest unit of text with
semantic significance.

=item tokener

A module that breaks a program text into a sequence of
L<tokens|/token> for later analysis by a parser.

=item tokenizing

Splitting up a program text into L<tokens|/token>.  Also known as
"lexing", in which case you get "lexemes" instead of tokens.

=item toolbox approach

The notion that, 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 transliterate

To turn one string representation into another by mapping each
character of the source string to its corresponding character in the
result string.  See
L<perlop/trE<sol>SEARCHLISTE<sol>REPLACEMENTLISTE<sol>cds>.

=item trigger

An event that causes a L</handler> to be run.

=item trinary

Not a stellar system with three stars, but an L</operator> taking
three L<operands|/operand>.  Sometimes pronounced L</ternary>.

=item troff

A venerable 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 scalar value that doesn't evaluate to 0 or C<"">.

=item truncating

Emptying a file of existing contents, either automatically when
opening a file for writing or explicitly via the
L<truncate|perlfunc/truncate> function.

=item type

See L</data type> and L</class>.

=item type casting

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

=item typed lexical

A L</lexical variable> that is declared with a L</class> type: C<my
Pony $bill>.

=item typedef

A type definition in the C language.

=item typeglob

Use of 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 L<perldata/Typeglobs and Filehandles>.

=item typemap

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

=back

=head2 U

=over 4

=item UDP

User Datagram Protocol, the typical way to send L<datagrams|/datagram>
over the Internet.

=item UID

A user ID.  Often used in the context of L</file> or L</process>
ownership.

=item umask

A mask of those L</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 L<umask|perlfunc/umask>
function.

=item unary operator

An operator with only one L</operand>, like C<!> or
L<chdir|perlfunc/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 comprising all the major character sets of the world,
more or less.  See L<http://www.unicode.org>.

=item Unix

A very large 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.

=back

=head2 V

=over 4

=item value

An actual piece of data, in contrast to all the variables, references,
keys, indexes, operators, and whatnot that you need to access the
value.

=item variable

A named storage location that can hold any of various kinds of
L</value>, as your program sees fit.

=item variable interpolation

The L</interpolation> of a scalar or array variable into a string.

=item variadic

Said of a L</function> that happily receives an indeterminate number
of L</actual arguments>.

=item vector

Mathematical jargon for a list of L<scalar values|/scalar value>.

=item virtual

Providing the appearance of something without the reality, as in:
virtual memory is not real memory.  (See also L</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 of L</scalar context> in which an L</expression> is not
expected to return any L</value> at all and is evaluated for its
L</side effects> alone.

=item v-string

A "version" or "vector" L</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 L</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 printed to the L</STDERR> stream to the effect that something
might be wrong but isn't worth blowing up over.  See L<perlfunc/warn>
and the L<warnings> pragma.

=item watch expression

An expression which, when its value changes, causes a breakpoint in
the Perl debugger.

=item whitespace

A L</character> 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.

=item word

In normal "computerese", the 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 L</identifier> (including underscores), or to a string of
nonwhitespace L<characters|/character> bounded by whitespace or string
boundaries.

=item working directory

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

=item wrapper

A program 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 You See Is What You Get.  Usually used when something that
appears on the screen matches how it will eventually look, like Perl's
L<format|perlfunc/format> declarations.  Also used to mean the
opposite of magic because everything works exactly as it appears, as
in the three-argument form of L<open|perlfunc/open>.

=back

=head2 X

=over 4

=item XS

An extraordinarily exported, expeditiously excellent, expressly
eXternal Subroutine, executed in existing C or C++ or in an exciting
new extension language called (exasperatingly) XS.  Examine L<perlxs>
for the exact explanation or L<perlxstut> for an exemplary unexacting
one.

=item XSUB

An external L</subroutine> defined in L</XS>.

=back

=head2 Y

=over 4

=item yacc

Yet 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 subpattern L</assertion> matching the L</null string> between
L<characters|/character>.

=item zombie

A process that has died (exited) but whose parent has not yet received
proper notification of its demise by virtue of having called
L<wait|perlfunc/wait> or L<waitpid|perlfunc/waitpid>.  If you
L<fork|perlfunc/fork>, you must clean up after your child processes
when they exit, or else 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 Programming Perl, Third Edition,
by Larry Wall, Tom Christiansen & Jon Orwant.
Copyright (c) 2000, 1996, 1991 O'Reilly Media, Inc.
This document may be distributed under the same terms as Perl itself.