=head1 NAME X X perlsub - Perl subroutines (user-defined functions) =head1 SYNOPSIS To declare subroutines: X X_{sub NAME; # A "forward" declaration.
sub NAME(PROTO); # ditto, but with prototypes
sub NAME : ATTRS; # with attributes
sub NAME(PROTO) : ATTRS; # with attributes and prototypes

sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
sub NAME : ATTRS BLOCK # with attributes
sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes

use feature 'signatures';
sub NAME(SIG) BLOCK # with signature
sub NAME :ATTRS (SIG) BLOCK # with signature, attributes
sub NAME :prototype(PROTO) (SIG) BLOCK # with signature, prototype

To define an anonymous subroutine at runtime:
X

$subref = sub BLOCK; # no proto
$subref = sub (PROTO) BLOCK; # with proto
$subref = sub : ATTRS BLOCK; # with attributes
$subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

use feature 'signatures';
$subref = sub (SIG) BLOCK; # with signature
$subref = sub : ATTRS(SIG) BLOCK; # with signature, attributes

To import subroutines:
X

use MODULE qw(NAME1 NAME2 NAME3);

To call subroutines:
X X

NAME(LIST); # Regular subroutine call.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME(LIST); # Circumvent prototypes.
&NAME; # Makes current @_ visible to called subroutine.

=head1 DESCRIPTION

Like many languages, Perl provides for user-defined subroutines.
These may be located anywhere in the main program, loaded in from
other files via the C, C, or C keywords, or
generated on the fly using C or anonymous subroutines.
You can even call a function indirectly using a variable containing
its name or a CODE reference.

The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
all functions likewise return to their caller one single flat list of
scalars. Any arrays or hashes in these call and return lists will
collapse, losing their identities--but you may always use
pass-by-reference instead to avoid this. Both call and return lists may
contain as many or as few scalar elements as you'd like. (Often a
function without an explicit return statement is called a subroutine, but
there's really no difference from Perl's perspective.)
X X

In a subroutine that uses signatures (see L below),
arguments are assigned into lexical variables introduced by the
signature. In the current implementation of Perl they are also
accessible in the C array in the same way as for non-signature
subroutines, but accessing them in this manner is now discouraged inside
such a signature-using subroutine.

In a subroutine that does not use signatures, any arguments passed in
show up in the array C. Therefore, if you called a function with
two arguments, those would be stored in C and C. The
array C is a local array, but its elements are aliases for the
actual scalar parameters. In particular, if an element C is
updated, the corresponding argument is updated (or an error occurs if it
is not updatable). If an argument is an array or hash element which did
not exist when the function was called, that element is created only
when (and if) it is modified or a reference to it is taken. (Some
earlier versions of Perl created the element whether or not the element
was assigned to.) Assigning to the whole array C removes that
aliasing, and does not update any arguments.
X X X

When not using signatures, Perl does not otherwise provide a means to
create named formal parameters. In practice all you do is assign to a
C list of these. Variables that aren't declared to be private are
global variables. For gory details on creating private variables, see
L"Private Variables via my()"> and L"Temporary Values via local()">.
To create protected environments for a set of functions in a separate
package (and probably a separate file), see L.

A C statement may be used to exit a subroutine, optionally
specifying the returned value, which will be evaluated in the
appropriate context (list, scalar, or void) depending on the context of
the subroutine call. If you specify no return value, the subroutine
returns an empty list in list context, the undefined value in scalar
context, or nothing in void context. If you return one or more
aggregates (arrays and hashes), these will be flattened together into
one large indistinguishable list.

If no C is found and if the last statement is an expression, its
value is returned. If the last statement is a loop control structure
like a C or a C, the returned value is unspecified. The
empty sub returns the empty list.
X X X

Example:

sub max {
my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max )) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last LINE;
}
}
return $thisline;
}

$lookahead = ; # get first line
while (defined($line = get_line())) {
...
}

Assigning to a list of private variables to name your arguments:

sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}

Because the assignment copies the values, this also has the effect
of turning call-by-reference into call-by-value. Otherwise a
function is free to do in-place modifications of C and change
its caller's values.
X X

upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}

You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception. For example, this won't work:
X X

upcase_in("frederick");

It would be much safer if the C function
were written to return a copy of its parameters instead
of changing them in place:

($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
sub upcase {
return unless defined wantarray; # void context, do nothing
my @parms = @_;
for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
}

Notice how this (unprototyped) function doesn't care whether it was
passed real scalars or arrays. Perl sees all arguments as one big,
long, flat parameter list in C. This is one area where
Perl's simple argument-passing style shines. The C
function would work perfectly well without changing the C
definition even if we fed it things like this:

@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );

Do not, however, be tempted to do this:

(@x, @y) = upcase(@list1, @list2);

Like the flattened incoming parameter list, the return list is also
flattened on return. So all you have managed to do here is stored
everything in C and made C empty. See
L for alternatives.

A subroutine may be called using an explicit C prefix. The
C is optional in modern Perl, as are parentheses if the
subroutine has been predeclared. The C is I optional
when just naming the subroutine, such as when it's used as
an argument to defined() or undef(). Nor is it optional when you
want to do an indirect subroutine call with a subroutine name or
reference using the C or C constructs,
although the C() >> notation solves that problem.
See L for more about all that.
X

Subroutines may be called recursively. If a subroutine is called
using the C form, the argument list is optional, and if omitted,
no C array is set up for the subroutine: the C array at the
time of the call is visible to subroutine instead. This is an
efficiency mechanism that new users may wish to avoid.
X

&foo(1,2,3); # pass three arguments
foo(1,2,3); # the same

foo(); # pass an empty argument list
&foo(); # the same

&foo; # foo() gets current args, like foo(@_)!
use strict 'subs';
foo; # like foo() iff sub foo predeclared, else
# a compile-time error
no strict 'subs';
foo; # like foo() iff sub foo predeclared, else
# a literal string "foo"

Not only does the C form make the argument list optional, it also
disables any prototype checking on arguments you do provide. This
is partly for historical reasons, and partly for having a convenient way
to cheat if you know what you're doing. See L below.
X

Since Perl 5.16.0, the C<__sub__> token is available under C and C. It will evaluate to a reference to the
currently-running sub, which allows for recursive calls without knowing
your subroutine's name.

use v5.16;
my $factorial = sub {
my ($x) = @_;
return 1 if $x == 1;
return($x * __SUB__->( $x - 1 ) );
};

The behavior of C<__sub__> within a regex code block (such as C(?{...})/>)
is subject to change.

Subroutines whose names are in all upper case are reserved to the Perl
core, as are modules whose names are in all lower case. A subroutine in
all capitals is a loosely-held convention meaning it will be called
indirectly by the run-time system itself, usually due to a triggered event.
Subroutines whose name start with a left parenthesis are also reserved the
same way. The following is a list of some subroutines that currently do
special, pre-defined things.

=over

=item documented later in this document

C

=item documented in L

C, C

=item documented in L

C, C

=item documented in L

C, C, C, C, C, C, C,
C, C, C, C, C, C,
C, C, C, C, C, C, C,
C, C, C, C, C, C,
C, C, C, C, C,
C, C, C, C

=item documented in L<:via>

C, C, C, C, C, C, C,
C, C, C, C, C, C, C,
C, C, C, C, C, C

=item documented in L

L|perlfunc/use >>, L|perlfunc/use >>,
L|perlfunc/require >>

=item documented in L

C

=item documented in L

C<:db>, C<:sub>, C<:lsub>, C<:goto>, C<:postponed>

=item undocumented, used internally by the L feature

any starting with C

=back

The C, C, C, C and C subroutines
are not so much subroutines as named special code blocks, of which you
can have more than one in a package, and which you can B call
explicitly. See L

=head2 Signatures

X X

Perl has a facility to allow a subroutine's formal parameters to be
declared by special syntax, separate from the procedural code of the
subroutine body. The formal parameter list is known as a I.

This facility must be enabled before it can be used. It is enabled
automatically by a C (or higher) declaration, or more
directly by C, in the current scope.

The signature is part of a subroutine's body. Normally the body of a
subroutine is simply a braced block of code, but when using a signature,
the signature is a parenthesised list that goes immediately before the
block, after any name or attributes.

For example,

sub foo :lvalue ($x, $y = 1, @z) { .... }

The signature declares lexical variables that are
in scope for the block. When the subroutine is called, the signature
takes control first. It populates the signature variables from the
list of arguments that were passed. If the argument list doesn't meet
the requirements of the signature, then it will throw an exception.
When the signature processing is complete, control passes to the block.

Positional parameters are handled by simply naming scalar variables in
the signature. For example,

sub foo ($left, $right) {
return $left + $right;
}

takes two positional parameters, which must be filled at runtime by
two arguments. By default the parameters are mandatory, and it is
not permitted to pass more arguments than expected. So the above is
equivalent to

sub foo {
die "Too many arguments for subroutine" unless @_ = 2;
my $left = $_[0];
my $right = $_[1];
return $left + $right;
}

An argument can be ignored by omitting the main part of the name from
a parameter declaration, leaving just a bare C sigil. For example,

sub foo ($first, $, $third) {
return "first=$first, third=$third";
}

Although the ignored argument doesn't go into a variable, it is still
mandatory for the caller to pass it.

A positional parameter is made optional by giving a default value,
separated from the parameter name by C:

sub foo ($left, $right = 0) {
return $left + $right;
}

The above subroutine may be called with either one or two arguments.
The default value expression is evaluated when the subroutine is called,
so it may provide different default values for different calls. It is
only evaluated if the argument was actually omitted from the call.
For example,

my $auto_id = 0;
sub foo ($thing, $id = $auto_id++) {
print "$thing has ID $id";
}

automatically assigns distinct sequential IDs to things for which no
ID was supplied by the caller. A default value expression may also
refer to parameters earlier in the signature, making the default for
one parameter vary according to the earlier parameters. For example,

sub foo ($first_name, $surname, $nickname = $first_name) {
print "$first_name $surname is known as \"$nickname\"";
}

Since Perl 5.38, a default value expression can also be written using the
C/=> operator, where it will be evaluated and used if the caller omitted
a value or the value provided was C.

sub foo ($name //= "world") {
print "Hello, $name";
}

foo(undef); # will print "Hello, world"

Similarly since Perl 5.38, the C operator can be used to provide a
default expression to be used whenever the caller provided a false value
(and remember that a missing or C value are also false).

sub foo ($x ||= 10) {
return 5 + $x;
}

An optional parameter can be nameless just like a mandatory parameter.
For example,

sub foo ($thing, $ = 1) {
print $thing;
}

The parameter's default value will still be evaluated if the corresponding
argument isn't supplied, even though the value won't be stored anywhere.
This is in case evaluating it has important side effects. However, it
will be evaluated in void context, so if it doesn't have side effects
and is not trivial it will generate a warning if the "void" warning
category is enabled. If a nameless optional parameter's default value
is not important, it may be omitted just as the parameter's name was:

sub foo ($thing, $=) {
print $thing;
}

Optional positional parameters must come after all mandatory positional
parameters. (If there are no mandatory positional parameters then the
optional positional parameters can be the first thing in the signature.)
If there are multiple optional positional parameters and not enough
arguments are supplied to fill them all, they will be filled from left
to right.

After positional parameters, additional arguments may be captured in a
slurpy parameter. The simplest form of this is just an array variable:

sub foo ($filter, @inputs) {
print $filter->($_) foreach @inputs;
}

With a slurpy parameter in the signature, there is no upper limit on how
many arguments may be passed. A slurpy array parameter may be nameless
just like a positional parameter, in which case its only effect is to
turn off the argument limit that would otherwise apply:

sub foo ($thing, @) {
print $thing;
}

A slurpy parameter may instead be a hash, in which case the arguments
available to it are interpreted as alternating keys and values.
There must be as many keys as values: if there is an odd argument then
an exception will be thrown. Keys will be stringified, and if there are
duplicates then the later instance takes precedence over the earlier,
as with standard hash construction.

sub foo ($filter, %inputs) {
print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
}

A slurpy hash parameter may be nameless just like other kinds of
parameter. It still insists that the number of arguments available to
it be even, even though they're not being put into a variable.

sub foo ($thing, %) {
print $thing;
}

A slurpy parameter, either array or hash, must be the last thing in the
signature. It may follow mandatory and optional positional parameters;
it may also be the only thing in the signature. Slurpy parameters cannot
have default values: if no arguments are supplied for them then you get
an empty array or empty hash.

A signature may be entirely empty, in which case all it does is check
that the caller passed no arguments:

sub foo () {
return 123;
}

Prior to Perl 5.36 these were considered experimental, and emitted a
warning in the C<:signatures> category. From Perl 5.36
onwards this no longer happens, though the warning category still exists
for back-compatibility with code that attempts to disable it with a
statement such as:

no warnings 'experimental::signatures';

In the current Perl implementation, when using a signature the arguments
are still also available in the special array variable C. However,
accessing them via this array is now discouraged, and should not be
relied upon in newly-written code as this ability may change in a future
version. Code that attempts to access the C array will produce
warnings in the C<:args_array_with_signatures> category when
compiled:

sub f ($x) {
# This line emits the warning seen below
print "Arguments are @_";
}

Z

Use of @_ in join or string with signatured subroutine is
experimental at ...

There is a difference between the two ways of accessing the arguments:
C I the arguments, but the signature variables get
I of the arguments. So writing to a signature variable only
changes that variable, and has no effect on the caller's variables, but
writing to an element of C modifies whatever the caller used to
supply that argument.

There is a potential syntactic ambiguity between signatures and prototypes
(see L), because both start with an opening parenthesis and
both can appear in some of the same places, such as just after the name
in a subroutine declaration. For historical reasons, when signatures
are not enabled, any opening parenthesis in such a context will trigger
very forgiving prototype parsing. Most signatures will be interpreted
as prototypes in those circumstances, but won't be valid prototypes.
(A valid prototype cannot contain any alphabetic character.) This will
lead to somewhat confusing error messages.

To avoid ambiguity, when signatures are enabled the special syntax
for prototypes is disabled. There is no attempt to guess whether a
parenthesised group was intended to be a prototype or a signature.
To give a subroutine a prototype under these circumstances, use a
L. For example,

sub foo :prototype($) { $_[0] }

It is entirely possible for a subroutine to have both a prototype and
a signature. They do different jobs: the prototype affects compilation
of calls to the subroutine, and the signature puts argument values into
lexical variables at runtime. You can therefore write

sub foo :prototype($$) ($left, $right) {
return $left + $right;
}

The prototype attribute, and any other attributes, must come before
the signature. The signature always immediately precedes the block of
the subroutine's body.

=head2 Private Variables via my()
X X X X X
X X

Synopsis:

my $foo; # declare $foo lexically local
my (@wid, %get); # declare list of variables local
my $foo = "flurp"; # declare $foo lexical, and init it
my @oof = @bar; # declare @oof lexical, and init it
my $x : Foo = $y; # similar, with an attribute applied

B: The use of attribute lists on C declarations is still
evolving. The current semantics and interface are subject to change.
See L and L<:handlers>.

The C operator declares the listed variables to be lexically
confined to the enclosing block, conditional
(C/C/C/C), loop
(C/C/C/C/C), subroutine, C,
or C/C/C'd file. If more than one value is listed, the
list must be placed in parentheses. All listed elements must be
legal lvalues. Only alphanumeric identifiers may be lexically
scoped--magical built-ins like C must currently be Cized
with C instead to limit their scope dynamically.

Unlike global or package variables localized by the C operator,
lexical variables declared with C are totally hidden from the outside
world, including any called subroutines. This is true if it's the same
subroutine called from itself or elsewhere--every call gets its own copy.
X

This doesn't mean that a C variable declared in a statically
enclosing lexical scope would be invisible. Only dynamic scopes
are cut off. For example, the C function below has access
to the lexical $x variable because both the C and the C_{occurred at the same scope, presumably file scope.

my $x = 10;
sub bumpx { $x++ }

An C, however, can see lexical variables of the scope it is
being evaluated in, so long as the names aren't hidden by declarations within
the C itself. See L.
X

The parameter list to my() may be assigned to if desired, which allows you
to initialize your variables. (If no initializer is given for a
particular variable, it is created with the undefined value.) Commonly
this is used to name input parameters to a subroutine. Examples:

$arg = "fred"; # "global" variable
$n = cube_root(27);
print "$arg thinks the root is $n\n";
# outputs: fred thinks the root is 3

sub cube_root {
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
}

The C is simply a modifier on something you might assign to. So when
you do assign to variables in its argument list, C doesn't
change whether those variables are viewed as a scalar or an array. So

my ($foo) = ; # WRONG?
my @FOO = ;

both supply a list context to the right-hand side, while

my $foo = ;

supplies a scalar context. But the following declares only one variable:

my $foo, $bar = 1; # WRONG

That has the same effect as

my $foo;
$bar = 1;

The declared variable is not introduced (is not visible) until after
the current statement. Thus,

my $x = $x;

can be used to initialize a new $x with the value of the old $x, and
the expression

my $x = 123 and $x == 123

is false unless the old $x happened to have the value C.

Lexical scopes of control structures are not bounded precisely by the
braces that delimit their controlled blocks; control expressions are
part of that scope, too. Thus in the loop

while (my $line = ) {
$line = lc $line;
} continue {
print $line;
}

the scope of $line extends from its declaration throughout the rest of
the loop construct (including the C clause), but not beyond
it. Similarly, in the conditional

if ((my $answer = ) =~ /^yes$/i) {
user_agrees();
} elsif ($answer =~ /^no$/i) {
user_disagrees();
} else {
chomp $answer;
die "'$answer' is neither 'yes' nor 'no'";
}

the scope of $answer extends from its declaration through the rest
of that conditional, including any C and C clauses,
but not beyond it. See L for information
on the scope of variables in statements with modifiers.

The C loop defaults to scoping its index variable dynamically
in the manner of C. However, if the index variable is
prefixed with the keyword C, or if there is already a lexical
by that name in scope, then a new lexical is created instead. Thus
in the loop
X X

for my $i (1, 2, 3) {
some_function();
}

the scope of $i extends to the end of the loop, but not beyond it,
rendering the value of $i inaccessible within C.
X X

Some users may wish to encourage the use of lexically scoped variables.
As an aid to catching implicit uses to package variables,
which are always global, if you say

use strict 'vars';

then any variable mentioned from there to the end of the enclosing
block must either refer to a lexical variable, be predeclared via
C or C, or else must be fully qualified with the package name.
A compilation error results otherwise. An inner block may countermand
this with C.

A C has both a compile-time and a run-time effect. At compile
time, the compiler takes notice of it. The principal usefulness
of this is to quiet C, but it is also essential
for generation of closures as detailed in L. Actual
initialization is delayed until run time, though, so it gets executed
at the appropriate time, such as each time through a loop, for
example.

Variables declared with C are not part of any package and are therefore
never fully qualified with the package name. In particular, you're not
allowed to try to make a package variable (or other global) lexical:

my $pack::var; # ERROR! Illegal syntax

In fact, a package or global variable is still accessible using the
fully qualified C<::> notation even while a lexical of the same name
is also visible:

package main;
our $x = 10;
my $x = 20;
print "$x and $::x\n";

That will print out C and C.

You may declare C variables at the outermost scope of a file
to hide any such identifiers from the world outside that file. This
is similar in spirit to C's static variables when they are used at
the file level. To do this with a subroutine requires the use of
a closure (an anonymous function that accesses enclosing lexicals).
If you want to create a private subroutine that cannot be called
from outside that block, it can declare a lexical variable containing
an anonymous sub reference:

my $secret_version = '1.001-beta';
my $secret_sub = sub { print $secret_version };
$secret_sub->();

As long as the reference is never returned by any function within the
module, no outside module can see the subroutine, because its name is not in
any package's symbol table. Remember that it's not I called
C or anything; it's just $secret_version,
unqualified and unqualifiable.

This does not work with object methods, however; all object methods
have to be in the symbol table of some package to be found. See
L for something of a work-around to
this.

=head2 Persistent Private Variables
X X X X
X X

There are two ways to build persistent private variables in Perl 5.10.
First, you can simply use the C feature. Or, you can use closures,
if you want to stay compatible with releases older than 5.10.

=head3 Persistent variables via state()

Beginning with Perl 5.10.0, you can declare variables with the C
keyword in place of C. For that to work, though, you must have
enabled that feature beforehand, either by using the C pragma, or
by using C on one-liners (see L). Beginning with Perl 5.16,
the C<:state> form does not require the
C pragma.

The C keyword creates a lexical variable (following the same scoping
rules as C) that persists from one subroutine call to the next. If a
state variable resides inside an anonymous subroutine, then each copy of
the subroutine has its own copy of the state variable. However, the value
of the state variable will still persist between calls to the same copy of
the anonymous subroutine. (Don't forget that C_{creates a new
subroutine each time it is executed.)

For example, the following code maintains a private counter, incremented
each time the gimme_another() function is called:

use feature 'state';
sub gimme_another { state $x; return ++$x }

And this example uses anonymous subroutines to create separate counters:

use feature 'state';
sub create_counter {
return sub { state $x; return ++$x }
}

Also, since C is lexical, it can't be reached or modified by any Perl
code outside.

When combined with variable declaration, simple assignment to C
variables (as in C) is executed only the first time. When such
statements are evaluated subsequent times, the assignment is ignored. The
behavior of assignment to C declarations where the left hand side
of the assignment involves any parentheses is currently undefined.

=head3 Persistent variables with closures

Just because a lexical variable is lexically (also called statically)
scoped to its enclosing block, C, or C FILE, this doesn't mean that
within a function it works like a C static. It normally works more
like a C auto, but with implicit garbage collection.

Unlike local variables in C or C++, Perl's lexical variables don't
necessarily get recycled just because their scope has exited.
If something more permanent is still aware of the lexical, it will
stick around. So long as something else references a lexical, that
lexical won't be freed--which is as it should be. You wouldn't want
memory being free until you were done using it, or kept around once you
were done. Automatic garbage collection takes care of this for you.

This means that you can pass back or save away references to lexical
variables, whereas to return a pointer to a C auto is a grave error.
It also gives us a way to simulate C's function statics. Here's a
mechanism for giving a function private variables with both lexical
scoping and a static lifetime. If you do want to create something like
C's static variables, just enclose the whole function in an extra block,
and put the static variable outside the function but in the block.

{
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
# $secret_val now becomes unreachable by the outside
# world, but retains its value between calls to gimme_another

If this function is being sourced in from a separate file
via C or C, then this is probably just fine. If it's
all in the main program, you'll need to arrange for the C
to be executed early, either by putting the whole block above
your main program, or more likely, placing merely a C
code block around it to make sure it gets executed before your program
starts to run:

BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}

See L about the
special triggered code blocks, C, C, C,
C and C.

If declared at the outermost scope (the file scope), then lexicals
work somewhat like C's file statics. They are available to all
functions in that same file declared below them, but are inaccessible
from outside that file. This strategy is sometimes used in modules
to create private variables that the whole module can see.

=head2 Temporary Values via local()
X X X X
X

B: In general, you should be using C instead of C, because
it's faster and safer. Exceptions to this include the global punctuation
variables, global filehandles and formats, and direct manipulation of the
Perl symbol table itself. C is mostly used when the current value
of a variable must be visible to called subroutines.

Synopsis:

# localization of values

local $foo; # make $foo dynamically local
local (@wid, %get); # make list of variables local
local $foo = "flurp"; # make $foo dynamic, and init it
local @oof = @bar; # make @oof dynamic, and init it

local $hash{key} = "val"; # sets a local value for this hash entry
delete local $hash{key}; # delete this entry for the current block
local ($cond ? $v1 : $v2); # several types of lvalues support
# localization

# localization of symbols

local *FH; # localize $FH, @FH, %FH, &FH ...
local *merlyn = *randal; # now $merlyn is really $randal, plus
# @merlyn is really @randal, etc
local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc

A C modifies its listed variables to be "local" to the
enclosing block, C, or C--and to I. A C just gives temporary
values to global (meaning package) variables. It does I create
a local variable. This is known as dynamic scoping. Lexical scoping
is done with C, which works more like C's auto declarations.

Some types of lvalues can be localized as well: hash and array elements
and slices, conditionals (provided that their result is always
localizable), and symbolic references. As for simple variables, this
creates new, dynamically scoped values.

If more than one variable or expression is given to C, they must be
placed in parentheses. This operator works
by saving the current values of those variables in its argument list on a
hidden stack and restoring them upon exiting the block, subroutine, or
eval. This means that called subroutines can also reference the local
variable, but not the global one. The argument list may be assigned to if
desired, which allows you to initialize your local variables. (If no
initializer is given for a particular variable, it is created with an
undefined value.)

Because C is a run-time operator, it gets executed each time
through a loop. Consequently, it's more efficient to localize your
variables outside the loop.

=head3 Grammatical note on local()
X

A C is simply a modifier on an lvalue expression. When you assign to
a Cized variable, the C doesn't change whether its list is
viewed as a scalar or an array. So

local($foo) = ;
local @FOO =}}}