PERLSUB(1)

💖 NAME 💖

perlsub — Perl subroutines (user-defined functions). It's the page about subs, bestie, and it's in the Perl Programmers Reference Guide (that's the manual it lives in, PERLSUB(1) energy).

---

✨ SYNOPSIS ✨

🎀 To declare subroutines:

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:

$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:

use MODULE qw(NAME1 NAME2 NAME3);

💅 To call subroutines:

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

---

🔥 DESCRIPTION 🔥

Like, so many languages out there, Perl gives you user-defined subroutines. Bestie, you can put them literally anywhere: in the main program, pull them in from other files with the do, require, or use keywords, or just spawn them on the fly with eval or anonymous subroutines. You can even call a function indirectly using a variable that holds its name or a CODE reference. Main character behaviour, honestly.

The Perl model for function call and return values is simple: every function gets passed, as parameters, one single flat list of scalars, and every function hands back to its caller one single flat list of scalars too. Any arrays or hashes hanging out in those call and return lists will collapse, losing their identities — but no worries, you can always go pass-by-reference instead to dodge that. Both the call list and the return list can hold as many or as few scalar elements as you want. (A function with no explicit return statement is often called a subroutine, but ngl from Perl's point of view there's genuinely no difference.)

In a subroutine that uses signatures (peep "Signatures" below), the arguments get assigned into lexical variables introduced by the signature. In the current implementation of Perl they're also accessible in the @_ array the same way as for non-signature subroutines, but accessing them that way is now discouraged inside a signature-using subroutine. Like, we said what we said.

In a subroutine that does not use signatures, any arguments passed in show up in the array @_. So if you called a function with two arguments, those would be chilling in $_[0] and $_[1]. The array @_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated too (or you get an error if it isn't updatable). If an argument is an array or hash element that did not exist when the function was called, that element gets created only when (and if) it's modified or a reference to it is taken. (Some earlier versions of Perl created the element whether or not it was assigned to — messy.) Assigning to the whole array @_ removes that aliasing, and does not update any arguments.

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

A return 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), those get flattened together into one big indistinguishable list. 💀

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

🍓 Example:

sub max {
    my $max = shift(@_);
    foreach $foo (@_) {
        $max = $foo if $max < $foo;
    }
    return $max;
}
$bestday = max($mon,$tue,$wed,$thu,$fri);

🌸 Example:

# get a line, combining continuation lines
#  that start with whitespace

sub get_line {
    $thisline = $lookahead;  # global variables!
    LINE: while (defined($lookahead = <STDIN>)) {
        if ($lookahead =~ /^[ \t]/) {
            $thisline .= $lookahead;
        }
        else {
            last LINE;
        }
    }
    return $thisline;
}

$lookahead = <STDIN>;       # get first line
while (defined($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 @_ and change its caller's values. Which, like, is a choice.

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 this way, obviously. 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:

upcase_in("frederick");

It would be much safer if the upcase_in() 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 @_. This is one area where Perl's simple argument-passing style shines. The upcase() function would work perfectly well without changing the upcase() 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 store everything in @x and make @y empty. 😭 See "Pass by Reference" for alternatives.

A subroutine may be called using an explicit & prefix. The & is optional in modern Perl, and so are parentheses if the subroutine has been predeclared. The & is not optional when you're just naming the subroutine, like 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 &$subref() or &{$subref}() constructs, although the $subref->() notation solves that problem. See perlref for more about all that.

Subroutines may be called recursively. If a subroutine is called using the & form, the argument list is optional, and if it's omitted, no @_ array is set up for the subroutine: the @_ array from the time of the call is visible to the subroutine instead. This is an efficiency mechanism that new users may wish to avoid. 🙈

&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 & 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 "Prototypes" below.

Since Perl 5.16.0, the __SUB__ token is available under use feature 'current_sub' and use v5.16. It will evaluate to a reference to the currently-running sub, which lets you do recursive calls without knowing your subroutine's name. Iconic.

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

The behavior of __SUB__ within a regex code block (such as /(?{...})/) is subject to change. Just so you know.

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.

• documented later in this document — AUTOLOAD
• documented in perlmod — CLONE, CLONE_SKIP
• documented in perlobj — DESTROY, DOES
• documented in perltie — the whole tie-your-objects-to-a-variable lineup, and yes you have to spell them in ALL CAPS like they're screaming:
  • "BINMODE", "CLEAR", "CLOSE", "DELETE", "DESTROY", "EOF", "EXISTS", "EXTEND", "FETCH", "FETCHSIZE", "FILENO", "FIRSTKEY", "GETC", "NEXTKEY", "OPEN", "POP", "PRINT", "PRINTF", "PUSH", "READ", "READLINE", "SCALAR", "SEEK", "SHIFT", "SPLICE", "STORE", "STORESIZE", "TELL", "TIEARRAY", "TIEHANDLE", "TIEHASH", "TIESCALAR", "UNSHIFT", "UNTIE", "WRITE"
• documented in PerlIO::via — the PerlIO layers gang, also fully capitalized bc of course:
  • "BINMODE", "CLEARERR", "CLOSE", "EOF", "ERROR", "FDOPEN", "FILENO", "FILL", "FLUSH", "OPEN", "POPPED", "PUSHED", "READ", "SEEK", "SETLINEBUF", "SYSOPEN", "TELL", "UNREAD", "UTF8", "WRITE"
• documented in perlfunc — lowercase besties, living their best life:
  • "import", "unimport", "INC"
• documented in UNIVERSAL:
  • "VERSION"
• documented in perldebguts — the debugger hooks, very niche, very cute:
  • "DB::DB", "DB::sub", "DB::lsub", "DB::goto", "DB::postponed"
• undocumented, used internally by the overload feature — sneaky little things, literally no docs:
  • any starting with "("

The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END" 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 not call explicitly. See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod.

---

✨ Signatures ✨

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 signature. 🎀

This facility must be enabled before it can be used. It is enabled automatically by a use v5.36 (or higher) declaration, or more directly by use feature 'signatures', in the current scope.

The signature is part of a subroutine's <em>body</em>. 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;
    die "Too few 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 $ 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. no cap, you still gotta hand it over even though nobody's catching it 🙈

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

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 "//=" operator, where it will be evaluated and used if the caller omitted a value or the value provided was "undef".

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

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

Similarly since Perl 5.38, the "||=" 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 "undef" 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 "experimental::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 @_. 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 @_ array will produce warnings in the "experimental::args_array_with_signatures" category when compiled:

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

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

There is a difference between the two ways of accessing the arguments: @_ <em>aliases</em> the arguments, but the signature variables get <em>copies</em> 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 @_ modifies whatever the caller used to supply that argument.

There is a potential syntactic ambiguity between signatures and prototypes (see "Prototypes"), 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 prototype attribute. 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.

---

💖 Private Variables via my() 💖

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

WARNING: bestie, the whole attribute lists on my declarations thing is still ✨evolving✨. Like, the current semantics and the interface are subject to change, okay?? Don't build your whole personality on it. Peep attributes and Attribute::Handlers for the deets. 🦋

---

The my operator declares the variables you list to be lexically confined to the enclosing block, conditional (if/unless/elsif/else), loop (for/foreach/while/until/continue), subroutine, eval, or do/require/use'd file. 🫶

If you're listing more than one value, the list must go in parentheses. Every element you list has to be a legal lvalue. And only alphanumeric identifiers can be lexically scoped — so those magical built-ins like $/? Nope, you still gotta localize those with local to limit their scope dynamically instead. 😭

Unlike global or package variables that got localized by the local operator, lexical variables declared with my are totally hidden from the outside world — including any subroutines you call. And that's true even if it's the same subroutine calling itself or getting called from somewhere else — every single call gets its own copy. periodt. 💅

Okay but don't get it twisted — this does not mean a my variable declared in a statically enclosing lexical scope is invisible. Only dynamic scopes get cut off. For example, the bumpx() function down here has access to the lexical $x variable because both the my and the sub happened at the same scope — presumably file scope:

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

An eval(), however, can see the lexical variables of the scope it's being evaluated in — as long as the names aren't hidden by declarations inside the eval() itself. See perlref. 👑

The parameter list to my() may be assigned to if you want, which is how you initialize your variables. (If you don't give an initializer for a particular variable, it gets created with the undefined value.) Usually this is how you name input parameters to a subroutine. Examples, bestie:

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

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

both of these give a list context to the right-hand side, while

my $foo = <STDIN>;

gives a scalar context. But this next one declares only one variable:

my $foo, $bar = 1;                  # WRONG

which has the exact same effect as:

my $foo;
$bar = 1;

The declared variable is not introduced (aka not visible) until after the current statement. So:

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 123. sneaky.

---

Lexical scopes of control structures are not bounded precisely by the braces that delimit their controlled blocks — the control expressions are part of that scope too. So in this loop:

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

the scope of $line extends from its declaration through the rest of the loop construct (including that continue clause), but not beyond it. Similarly, in this conditional:

if ((my $answer = <STDIN>) =~ /^yes$/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 elsif and else clauses — but not beyond it. Peep "Simple Statements" in perlsyn for the tea on variable scope in statements with modifiers. 🍓

The foreach loop defaults to scoping its index variable dynamically, in the manner of local. BUT — if the index variable is prefixed with the keyword my, or if there's already a lexical by that name in scope, then a new lexical gets created instead. So in this loop:

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

the scope of $i extends to the end of the loop, but not beyond it — which means the value of $i is inaccessible inside some_function(). 🙈

---

Some users may wanna encourage the use of lexically scoped variables. As a lil aid for catching implicit uses of package variables (which are always global), if you say

use strict 'vars';

then any variable mentioned from that point to the end of the enclosing block must either refer to a lexical variable, be predeclared via our or use vars, or else be fully qualified with the package name. Otherwise you get a compilation error. An inner block can countermand this with no strict 'vars'. ✨

A my has both a compile-time and a run-time effect. At compile time, the compiler takes notice of it. The main usefulness of that is to quiet use strict 'vars', but it's also essential for generating closures, as detailed in perlref. The actual initialization is delayed until run time, though — so it gets executed at the appropriate time, like each time through a loop, for example. 🔥

Variables declared with my are not part of any package, and are therefore never fully qualified with the package name. In particular, you are 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 :: 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 one prints out 20 and 10. 💅

You may declare my 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're used at the file level. To do this with a subroutine requires a closure (an anonymous function that accesses enclosing lexicals). If you wanna create a private subroutine that can't be called from outside that block, it can declare a lexical variable holding 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 isn't in any package's symbol table. Remember, it's not REALLY called $some_pack::secret_version or anything — it's just $secret_version, unqualified and unqualifiable. 🎀

This does not work with object methods, though — all object methods have to be in the symbol table of some package to be found. See "Function Templates" in perlref for something of a work-around to this.

---

🌸 Persistent Private Variables 🌸

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

✨ Persistent variables via state() ✨

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

The state keyword creates a lexical variable (following the same scoping rules as my) that persists from one subroutine call to the next. If a state variable lives 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. (And don't forget — sub { ... } creates a new subroutine each time it's executed!) 😭

For example, this code maintains a private counter, incremented each time the gimme_another() function gets 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 $x is lexical, it can't be reached or modified by any Perl code outside. locked away fr fr 🔒

When combined with variable declaration, simple assignment to state variables (like in state $x = 42) is executed only the first time. When such statements are evaluated on subsequent times, the assignment is ignored. The behavior of assignment to state declarations where the left hand side of the assignment involves any parentheses is currently undefined. ⚠️

🌈 Persistent variables with closures 🌈

Just because a lexical variable is lexically (also called statically) scoped to its enclosing block, eval, or do 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 exactly how it should be. You wouldn't want memory being freed until you were done using it, or kept around once you were done. Automatic garbage collection takes care of this for you. ✨

This means you can pass back or save away references to lexical variables, whereas returning 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

so like, $secret_val is now literally unreachable from the outside world, but it keeps its value between calls to gimme_another. that's the whole point, bestie 💅

If this function is being pulled in from a separate file via require or use, then honestly you're probably just fine ✨. But if it's all chillin' in the main program, you gotta make sure that my actually runs early — either by yeeting the whole block above your main program, or (way more likely) just wrapping it in a BEGIN code block so it's guaranteed to execute before your program starts running:

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

for the full tea on those special triggered code blocks — BEGIN, UNITCHECK, CHECK, INIT and END — go read "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod. 📚

If you declare them at the outermost scope (the file scope), then lexicals act kinda like C's file statics 🎀. They're available to every function in that same file declared below them, but nobody outside that file can touch them. Modules love this trick for creating private variables the whole module can see. 👑

🌸 Temporary Values via local() 🌸

WARNING: in general, you should be using my instead of local, because it's faster and safer. the exceptions to this include the global punctuation variables, global filehandles and formats, and direct manipulation of the Perl symbol table itself. local is mostly used when the current value of a variable has to be visible to called subroutines. 😤

Synopsis, bestie:

# 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 local makes the variables you list "local" to the enclosing block, eval, or do FILE — and also to any subroutine called from inside that block. local just hands out temporary values to global (meaning package) variables. It does not create a local variable. This is what's known as dynamic scoping. Lexical scoping is what my does, and it works more like C's auto declarations. 🧚

Some kinds of lvalues can be localized too: hash and array elements and slices, conditionals (as long as their result is always localizable), and symbolic references. Same deal as simple variables — this creates new, dynamically scoped values. ✨

If you hand local more than one variable or expression, they must go in parentheses. The way it works: it saves the current values of those variables in its argument list on a hidden stack, then restores them when you exit the block, subroutine, or eval. Which means called subroutines can also reference the local variable, but not the global one. And yeah, the argument list can be assigned to if you want, which lets you initialize your local variables. (If you don't give an initializer for a particular variable, it gets created with an undefined value.) 👀

Because local is a run-time operator, it gets executed every single time through a loop. So it's more efficient to localize your variables outside the loop. no cap. 🏃‍♀️

🎀 Grammatical note on local() 🎀

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

local($foo) = <STDIN>;
local @FOO = <STDIN>;

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

local $foo = <STDIN>;

supplies a scalar context. 💅

🔮 Localization of special variables 🔮

If you localize a special variable, you'll be giving it a new value, but its magic won't go away. Which means all the side-effects related to that magic still work with the localized value. ✨

That's what lets code like this cook:

# Read the whole contents of FILE in $slurp
{ local $/ = undef; $slurp = <FILE>; }

but heads up though — this does restrict localization of some values 😭. for example, as of Perl 5.10.0, the following statement dies with the error Modification of a read-only value attempted, because the $1 variable is magical and read-only:

local $1 = 2;

One exception is the default scalar variable: starting with Perl 5.14, local($_) will always strip all magic from $_, which makes it possible to safely reuse $_ inside a subroutine. 🫶

WARNING: localization of tied arrays and hashes does not currently work as described 😵. This will be fixed in a future release of Perl; in the meantime, avoid code that relies on any particular behavior of localising tied arrays or hashes (localising individual elements is still okay). See "Localising Tied Arrays and Hashes Is Broken" in perl58delta for more details.

🦋 Localization of globs 🦋

The construct:

local *name;

creates a whole new symbol table entry for the glob name in the current package. That means every variable in its glob slot — $name, @name, %name, &name, and the name filehandle — gets dynamically reset. 🔥

Among other things, this implies that any magic those variables eventually carry gets locally lost. In other words, saying local */ will not have any effect on the internal value of the input record separator. 💀

🍓 Localization of elements of composite types 🍓

Also worth taking a sec to explain what actually happens when you localize a member of a composite type (i.e. an array or hash element) 👑. In this case, the element is localized by name. Which means when the scope of the local() ends, the saved value gets restored to the hash element whose key was named in the local(), or to the array element whose index was named in the local(). And if that element got deleted while the local() was in effect (like by a delete() from a hash, or a shift() of an array), it will spring back into existence, possibly extending an array and filling in the skipped elements with undef. 😳

For instance, if you say:

%hash = ( 'This' => 'is', 'a' => 'test' );
@ary  = ( 0..5 );
{
    local($ary[5]) = 6;
    local($hash{'a'}) = 'drill';
    while (my $e = pop(@ary)) {
        print "$e . . .\n";
        last unless $e > 3;
    }
    if (@ary) {
        $hash{'only a'} = 'test';
        delete $hash{'a'};
    }
}
print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
print "The array has ",scalar(@ary)," elements: ",
    join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

Perl will print:

6 . . .
4 . . .
3 . . .
This is a test only a test.
The array has 6 elements: 0, 1, 2, undef, undef, 5

The behavior of local() on non-existent members of composite types is subject to change in future. And the behavior of local() on array elements specified using negative indexes is especially surprising, and is very likely to change. 😬

💜 Localized deletion of elements of composite types 💜

You can use the delete local $array[$idx] and delete local $hash{key} constructs to delete a composite type entry for the current block and then restore it when that block ends ✨. They return the array/hash value from before the localization, which means they're respectively equivalent to:

do {
    my $val = $array[$idx];
    local  $array[$idx];
    delete $array[$idx];
    $val
}

and:

do {
    my $val = $hash{key};
    local  $hash{key};
    delete $hash{key};
    $val
}

except that for those, the local is scoped to the do block. Slices are also accepted 🎀:

my %hash = (
    a => [ 7, 8, 9 ],
    b => 1,
)

{
    my $x = delete local $hash{a};
    # $x is [ 7, 8, 9 ]
    # %hash is (b => 1)

    {
        my @nums = delete local @$x[0, 2]
        # @nums is (7, 9)
        # $x is [ undef, 8 ]

        $x[0] = 999; # will be erased when the scope ends
    }
    # $x is back to [ 7, 8, 9 ]

}
# %hash is back to its original state

This construct is supported since Perl v5.12. 🫶

👑 Lvalue subroutines 👑

It is possible to return a modifiable value from a subroutine 😌. To do that, you have to declare the subroutine to return an lvalue:

my $val;
sub canmod : lvalue {
    $val;  # or:  return $val;
}
sub nomod {
    $val;
}

canmod() = 5;   # assigns to $val
nomod()  = 5;   # ERROR

The scalar/list context for the subroutine and for the right-hand side of the assignment is determined as if the subroutine call is replaced by a scalar. For example, consider:

data(2,3) = get_data(3,4);

Both subroutines here are called in a scalar context, while in:

(data(2,3)) = get_data(3,4);

and in:

(data(2),data(3)) = get_data(3,4);

all the subroutines are called in a list context. 💅

Lvalue subroutines are convenient, but you have to keep in mind that when they're used with objects, they may violate encapsulation 😭. A normal mutator can check the supplied argument before setting the attribute it's protecting; an lvalue subroutine cannot. So if you require any special processing when storing and retrieving the values, consider using the CPAN module Sentinel or something similar. 🫶

✨ Lexical Subroutines ✨

Beginning with Perl 5.18, you can declare a private subroutine with my or state 🎀. And just like with state variables, the state keyword is only available under use feature 'state' or use v5.10 or higher.

Prior to Perl 5.26, lexical subroutines were deemed experimental and were available only under the use feature 'lexical_subs' pragma. They also produced a warning unless the experimental::lexical_subs warnings category was disabled. 🙈

These subroutines are only visible within the block in which they are declared, and only after that declaration:

# Include these two lines if your code is intended to run under Perl
# versions earlier than 5.26.
no warnings "experimental::lexical_subs";
use feature 'lexical_subs';

foo();              # calls the package/global subroutine
state sub foo {
    foo();          # also calls the package subroutine
}
foo();              # calls "state" sub
my $ref = \&foo;    # take a reference to "state" sub

my sub bar { ... }
bar();              # calls "my" sub

okay so real talk bestie: you can't (directly) write a recursive lexical subroutine. here's the wrong way to try it, and the way it fails is honestly so rude 😭

# WRONG
my sub baz {
    baz();
}

This fails because baz() points at the package/global sub "baz", not the lexical sub you're currently defining. it's giving identity crisis 💀

The solution is to use "__SUB__", periodt ✨

my sub baz {
    __SUB__->();    # calls itself
}

It is possible to predeclare a lexical subroutine. The "sub foo {...}" subroutine definition syntax respects any previous "my sub;" or "state sub;" declaration. But using this to define recursive subroutines is a bad idea, no cap 🙅‍♀️

my sub baz;         # predeclaration
sub baz {           # define the "my" sub
    baz();          # WRONG: calls itself, but leaks memory
}

Just like "my $f; $f = sub { $f->() }", this example leaks memory. The name "baz" is a reference to the subroutine, and the subroutine uses the name "baz"; they keep each other alive (see "Circular References" in perlref). it's giving toxic relationship 💀

---

🎀 "state sub" vs "my sub" 🎀

What is the difference between "state" subs and "my" subs? Each time that execution enters a block when "my" subs are declared, a new copy of each sub is created. "State" subroutines persist from one execution of the containing block to the next. main character energy honestly 👑

So, in general, "state" subroutines are faster. But "my" subs are necessary if you want to create closures:

sub whatever {
    my $x = shift;
    my sub inner {
        ... do something with $x ...
    }
    inner();
}

In this example, a new $x is created when "whatever" is called, and also a new "inner", which can see the new $x. A "state" sub will only see the $x from the first call to "whatever". periodt.

---

🦋 "our" subroutines 🦋

Like "our $variable", "our sub" creates a lexical alias to the package subroutine of the same name. iconic behavior 💅

The two main uses for this are to switch back to using the package sub inside an inner scope:

sub foo { ... }

sub bar {
    my sub foo { ... }
    {
        # need to use the outer foo here
        our sub foo;
        foo();
    }
}

and to make a subroutine visible to other packages in the same scope:

package MySneakyModule;

our sub do_something { ... }

sub do_something_with_caller {
    package DB;
    () = caller 1;          # sets @DB::args
    do_something(@args);    # uses MySneakyModule::do_something
}

---

💖 Passing Symbol Table Entries (typeglobs) 💖

⚠️ WARNING: The mechanism described in this section was originally the only way to simulate pass-by-reference in older versions of Perl. While it still works fine in modern versions, the new reference mechanism is generally easier to work with. See below, bestie. 💅

Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so that the subroutine can modify the global copy of it rather than working with a local copy. In Perl you can refer to all objects of a particular name by prefixing the name with a star: *foo. This is often known as a "typeglob", because the star on the front can be thought of as a wildcard match for all the funny prefix characters on variables and subroutines and such. iykyk ⭐

When evaluated, the typeglob produces a scalar value that represents all the objects of that name, including any filehandle, format, or subroutine. When assigned to, it causes the name mentioned to refer to whatever "*" value was assigned to it. Example:

sub doubleary {
    local(*someary) = @_;
    foreach $elem (@someary) {
        $elem *= 2;
    }
}
doubleary(*foo);
doubleary(*bar);

Scalars are already passed by reference, so you can modify scalar arguments without using this mechanism by referring explicitly to $_[0] etc. You can modify all the elements of an array by passing all the elements as scalars, but you have to use the "*" mechanism (or the equivalent reference mechanism) to "push", "pop", or change the size of an array. It will certainly be faster to pass the typeglob (or reference).

Even if you don't want to modify an array, this mechanism is useful for passing multiple arrays in a single LIST, because normally the LIST mechanism will merge all the array values so that you can't extract out the individual arrays. For more on typeglobs, see "Typeglobs and Filehandles" in perldata.

---

🌈 When to Still Use local() 🌈

Despite the existence of "my", there are still three places where the "local" operator still shines. In fact, in these three places, you must use "local" instead of "my". no cap.

1. You need to give a global variable a temporary value, especially $_.

The global variables, like @ARGV or the punctuation variables, must be "local"ized with local(). This block reads in /etc/motd, and splits it up into chunks separated by lines of equal signs, which are placed in @Fields.

{
    local @ARGV = ("/etc/motd");
    local $/ = undef;
    local $_ = <>;
    @Fields = split /^\s*=+\s*$/;
}

In particular, it's important to "local"ize $_ in any routine that assigns to it. Look out for implicit assignments in "while" conditionals. 👀

2. You need to create a local file or directory handle or a local function.

A function that needs a filehandle of its own must use local() on a complete typeglob. This can be used to create new symbol table entries:

sub ioqueue {
    local  (*READER, *WRITER);    # not my!
    pipe    (READER,  WRITER)     or die "pipe: $!";
    return (*READER, *WRITER);
}
($head, $tail) = ioqueue();

See the Symbol module for a way to create anonymous symbol table entries. 🧚

Because assignment of a reference to a typeglob creates an alias, this can be used to create what is effectively a local function, or at least, a local alias.

{
    local *grow = \&shrink; # only until this block exits
    grow();                # really calls shrink()
    move();                # if move() grow()s, it shrink()s too
}
grow();                    # get the real grow() again

See "Function Templates" in perlref for more about manipulating functions by name in this way. 💜

3. You want to temporarily change just one element of an array or hash.

You can "local"ize just one element of an aggregate. Usually this is done on dynamics:

{
    local $SIG{INT} = 'IGNORE';
    funct();                            # uninterruptible
}
# interruptibility automatically restored here

But it also works on lexically declared aggregates. slay.

---

💅 Pass by Reference 💅

If you want to pass more than one array or hash into a function--or return them from it--and have them maintain their integrity, then you're going to have to use an explicit pass-by-reference. Before you do that, you need to understand references as detailed in perlref. This section may not make much sense to you otherwise. fr fr.

Here are a few simple examples. First, let's pass in several arrays to a function and have it "pop" all of them, returning a new list of all their former last elements:

@tailings = popmany ( \@w, \@x, \@y, \@z );

sub popmany {
    my $aref;
    my @retlist;
    foreach $aref ( @_ ) {
        push @retlist, pop @$aref;
    }
    return @retlist;
}

Here's how you might write a function that returns a list of keys occurring in all the hashes passed to it:

@common = inter( \%foo, \%bar, \%joe );
sub inter {
    my ($k, $href, %seen); # locals
    foreach $href (@_) {
        while ( $k = each %$href ) {
            $seen{$k}++;
        }
    }
    return grep { $seen{$_} == @_ } keys %seen;
}

So far, we're using just the normal list return mechanism. What happens if you want to pass or return a hash? Well, if you're using only one of them, or you don't mind them concatenating, then the normal calling convention is ok, although a little expensive. 💸

Where people get into trouble is here:

(@w, @x) = func(@y, @z);

or

(%w, %x) = func(%y, %z);

That syntax simply won't work. It sets just @w or %w and clears the @x or %x. Plus the function didn't get passed into two separate arrays or hashes: it got one long list in @_, as always. 💀

If you can arrange for everyone to deal with this through references, it's cleaner code, although not so nice to look at. Here's a function that takes two array references as arguments, returning the two array elements in order of how many elements they have in them:

($wref, $xref) = func(\@y, \@z);
print "@$wref has more than @$xref\n";
sub func {
    my ($yref, $zref) = @_;
    if (@$yref > @$zref) {
        return ($yref, $zref);
    } else {
        return ($zref, $yref);
    }
}

It turns out that you can actually do this also:

(*w, *x) = func(\@y, \@z);
print "@w has more than @x\n";
sub func {
    local (*y, *z) = @_;
    if (@y > @z) {
        return (\@y, \@z);
    } else {
        return (\@z, \@y);
    }
}

Here we're using the typeglobs to do symbol table aliasing. It's a tad subtle, though, and also won't work if you're using "my" variables, because only globals (even in disguise as "local"s) are in the symbol table. periodt.

If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references work, too. For example:

splutter(\*STDOUT);
sub splutter {
    my $fh = shift;
    print $fh "her um well a hmmm\n";
}

$rec = get_rec(\*STDIN);
sub get_rec {
    my $fh = shift;
    return scalar <$fh>;
}

If you're planning on generating new filehandles, you could do this. Notice to pass back just the bare *FH, not its reference.

sub openit {
    my $path = shift;
    local *FH;
    return open (FH, $path) ? *FH : undef;
}

---

✨ Prototypes ✨

okay bestie so Perl does support compile-time argument checking, but it's a very limited kind of situation, and it's called function prototyping. You can declare it in either the PROTO section or with a prototype attribute. So if you write either of these:

sub mypush (\@@)
sub mypush :prototype(\@@)

then mypush() takes arguments exactly like push() does. slay.

But wait — if subroutine signatures are enabled (peep "Signatures"), then the shorter PROTO syntax is not available, because it would beef with signatures. In that case your prototype can only be declared as an attribute. okurrr.

The function declaration must be visible at compile time. And the prototype only affects how regular calls to the function get interpreted — and "regular" here means not using the & sigil. Like, if you call it like a built-in function, it behaves like a built-in function. If you call it like an old-school (perl4) subroutine, it behaves like an old-school subroutine. And naturally falling out of that rule: prototypes have zero influence on subroutine references like \&foo, or on indirect subroutine calls like &{$subref}() or $subref->(). nope, none, not today.

Method calls aren't influenced by prototypes either, because the function being called is literally indeterminate at compile time — the exact code called depends on inheritance. iykyk.

So the whole intent of this feature is primarily to let you define subroutines that work like built-in functions. Here are prototypes for some other functions that parse almost exactly like their corresponding built-in:

Declared as             Called as

sub mylink ($$)         mylink $old, $new
sub myvec ($$$)         myvec $var, $offset, 1
sub myindex ($$;$)      myindex getstring(), "substr"
sub mysyswrite ($$$;$)  mysyswrite $buf, 0, length($buf) - $off, $off
sub myreverse (@)       myreverse $x, $y, $z
sub myjoin ($@)         myjoin ":", $x, $y, $z
sub mypop (\@)          mypop @array
sub mysplice (\@$$@)    mysplice @array, 0, 2, @pushme
sub mykeys (\[%@])      mykeys $hashref->%*
sub myopen (*;$)        myopen HANDLE, $name
sub mypipe (**)         mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@)         mygrep { /foo/ } $x, $y, $z
sub myrand (;$)         myrand 42
sub mytime ()           mytime

Any backslashed prototype character represents an actual argument that must start with that character (optionally preceded by my, our or local) — with the exception of $, which will accept any scalar lvalue expression, like $foo = 7 or my_function()->[0]. The value passed as part of @_ will be a reference to the actual argument given in the subroutine call, obtained by applying \ to that argument. cute.

You can also use the \[] backslash group notation to specify more than one allowed argument type. Like:

sub myref (\[$@%&*])

will allow calling myref() as

myref $var
myref @array
myref %hash
myref &sub
myref *glob

and the first argument of myref() will be a reference to a scalar, an array, a hash, a subroutine, or a glob. one function, five vibes 💅

Now, unbackslashed prototype characters have special meanings of their own:

• Any unbackslashed @ or % eats all the remaining arguments, and forces list context.
• An argument represented by $ forces scalar context.
• An & requires an anonymous subroutine, which — if passed as the first argument — may look like a bare block. It does not require the sub keyword or a subsequent comma.
• A * lets the subroutine accept a bareword, constant, scalar expression, typeglob, or a reference to a typeglob in that slot. The value will be available to the subroutine either as a simple scalar, or (in the latter two cases) as a reference to the typeglob. If you want to always convert such arguments to a typeglob reference, use Symbol::qualify_to_ref() like this:

use Symbol 'qualify_to_ref';

sub foo (*) {
    my $fh = qualify_to_ref(shift, caller);
    ...
}

The + prototype is a special alternative to $ that acts like \[@%] when you hand it a literal array or hash variable, but otherwise forces scalar context on the argument. This is so useful for functions that should accept either a literal array or an array reference as the argument:

sub mypush (+@) {
    my $aref = shift;
    die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
    push @$aref, @_;
}

When you use the + prototype, your function must check that the argument is of an acceptable type. no cap, that part's on you.

A semicolon (;) separates mandatory arguments from optional arguments. It's redundant before @ or %, since those two gobble up everything else anyway.

And as the last character of a prototype, or just before a semicolon, a @ or a %, you can use _ in place of $: if that argument isn't provided, $_ will be used instead. ✨

Notice how the last three examples in the table above get treated specially by the parser. mygrep() is parsed as a true list operator, myrand() is parsed as a true unary operator with unary precedence the same as rand(), and mytime() is truly without arguments, just like time(). So if you say

mytime +2;

you'll get mytime() + 2, not mytime(2) — which is how it would be parsed without a prototype. If you want to force a unary function to have the same precedence as a list operator, add ; to the end of the prototype:

sub mygetprotobynumber($;);
mygetprotobynumber $x > $y; # parsed as mygetprotobynumber($x > $y)

The interesting thing about & is that you can literally generate new syntax with it, as long as it's in the initial position:

sub try (&@) {
    my($try,$catch) = @_;
    eval { &$try };
    if ($@) {
        local $_ = $@;
        &$catch;
    }
}
sub catch (&) { $_[0] }

try {
    die "phooey";
} catch {
    /phooey/ and print "unphooey\n";
};

That prints unphooey. (Yes, there are still unresolved issues having to do with visibility of @_. I'm ignoring that question for the moment. (But note that if we make @_ lexically scoped, those anonymous subroutines can act like closures... (Gee, is this sounding a little Lispish? (Never mind.)))) 😭 the nesting goes so deep bestie

And here's a reimplementation of the Perl grep operator:

sub mygrep (&@) {
    my $code = shift;
    my @result;
    foreach $_ (@_) {
        push(@result, $_) if &$code;
    }
    @result;
}

Some folks would prefer full alphanumeric prototypes. But alphanumerics have been intentionally left out of prototypes for the express purpose of someday in the future adding named, formal parameters. The current mechanism's main goal is to let module writers provide better diagnostics for module users. Larry feels the notation is quite understandable to Perl programmers, and that it won't intrude greatly upon the meat of the module, nor make it harder to read. The line noise is visually encapsulated into a small pill that's easy to swallow. 💊 ate, honestly.

If you try to use an alphanumeric sequence in a prototype you'll generate an optional warning — Illegal character in prototype.... Unfortunately earlier versions of Perl allowed the prototype to be used as long as its prefix was a valid prototype. The warning may be upgraded to a fatal error in a future version of Perl, once the majority of offending code is fixed. so like, heads up.

It's probably best to prototype new functions, not retrofit prototyping into older ones. Why? Because you have to be especially careful about silent impositions of differing list versus scalar contexts. For example, if you decide a function should take just one parameter, like this:

sub func ($) {
    my $n = shift;
    print "you gave me $n\n";
}

and someone has been calling it with an array or an expression returning a list:

func(@foo);
func( $text =~ /\w+/g );

Then you've just supplied an automatic scalar in front of their argument, which can be more than a bit surprising. 😭 The old @foo that used to hold one thing doesn't get passed in. Instead, func() now gets passed in a 1; that is, the number of elements in @foo. And the m//g gets called in scalar context, so instead of a list of words it returns a boolean result and advances pos($text). Ouch! 💀

If a sub has both a PROTO and a BLOCK, the prototype is not applied until after the BLOCK is completely defined. Which means a recursive function with a prototype has to be predeclared for the prototype to take effect, like so:

sub foo($$);
sub foo($$) {
    foo 1, 2;
}

This is all very powerful, of course, and should be used only in moderation to make the world a better place. 🌸

---

🎀 Constant Functions 🎀

Functions with a prototype of () are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it will be used in place of function calls made without &. Calls made using & are never inlined. (See constant for an easy way to declare most constants.)

The following functions would all be inlined:

sub pi ()           { 3.14159 }             # Not exact, but close.
sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets,
                                            # and it's inlined, too!
sub ST_DEV ()       { 0 }
sub ST_INO ()       { 1 }

sub FLAG_FOO ()     { 1 << 8 }
sub FLAG_BAR ()     { 1 << 9 }
sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

sub N () { int(OPT_BAZ) / 3 }

sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } }

Be aware that the last example was not always inlined in Perl 5.20 and earlier, which did not behave consistently with subroutines containing inner scopes. 🙈

You can countermand inlining by using an explicit return:

sub baz_val () {
    if (OPT_BAZ) {
        return 23;
    }
    else {
        return 42;
    }
}
sub bonk_val () { return 12345 }

As alluded to earlier, you can also declare inlined subs dynamically at BEGIN time if their body consists of a lexically-scoped scalar which has no other references. Only the first example here will be inlined:

BEGIN {
    my $var = 1;
    no strict 'refs';
    *INLINED = sub () { $var };
}

BEGIN {
    my $var = 1;
    my $ref = \$var;
    no strict 'refs';
    *NOT_INLINED = sub () { $var };
}

okay bestie, this next caveat is genuinely NOT obvious — like, it has its own ticket and everything ([RT #79908]). It's about what happens when that variable is potentially modifiable. Watch this:

BEGIN {
    my $x = 10;
    *FOO = sub () { $x };
    $x++;
}
print FOO(); # printed 10 prior to 5.32.0

From Perl 5.22 onwards this handed you a deprecation warning, and from Perl 5.32 onwards it's a full-on run-time error. 💀 Back in the day the variable got inlined immediately and just… stopped behaving like a normal lexical variable, which is why it printed 10 and not 11. the way that just quietly changed the answer?? not it.

So if you still want a sub like that to be inlined (with no warning), make sure the variable isn't used anywhere it could get modified outside of where it's declared. ✨

# Fine, no warning
BEGIN {
    my $x = 54321;
    *INLINED = sub () { $x };
}
# Error
BEGIN {
    my $x;
    $x = 54321;
    *ALSO_INLINED = sub () { $x };
}

---

Perl 5.22 also dropped the "const" attribute as an alternative, iykyk. It started out experimental but got made stable in Perl 5.40. 🎀 When you slap it on an anonymous subroutine, it forces the sub to be called the moment the "sub" expression is evaluated. The return value gets captured and turned into a constant subroutine:

my $x = 54321;
*INLINED = sub : const { $x };
$x++;

The return value of "INLINED" in this example will always be 54321, no matter what you do to $x afterwards. obsessed. 💅 You can also put literally any arbitrary code inside the sub — it gets executed immediately and its return value captured the exact same way.

If you really want a subroutine with a "()" prototype that returns a lexical variable, you can force it to not be inlined super easily by adding an explicit "return":

BEGIN {
    my $x = 10;
    *FOO = sub () { return $x };
    $x++;
}
print FOO(); # prints 11

The easiest way to tell if a subroutine was inlined is by using B::Deparse. Check out this example of two subroutines both returning 1 — one with a "()" prototype that causes it to be inlined, and one without (deparse output truncated for clarity, bestie):

$ perl -MO=Deparse -e 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
sub ONE {
    1;
}
if (ONE ) {
    print ONE() if ONE ;
}

$ perl -MO=Deparse -e 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
sub ONE () { 1 }
do {
    print 1
};

If you redefine a subroutine that was eligible for inlining, you get a warning by default. 🙈 And you can use this warning to tell whether a particular subroutine counts as inlinable, because it's different from the warning you get for overriding non-inlined subroutines:

$ perl -e 'sub one () {1} sub one () {2}'
Constant subroutine one redefined at -e line 1.
$ perl -we 'sub one {1} sub one {2}'
Subroutine one redefined at -e line 1.

That warning is considered severe enough that it is not affected by the -w switch (or the absence of it), because previously compiled invocations of the function will still be using the old value of the function. So if you need to be able to redefine the subroutine, you have to make sure it isn't inlined — either by dropping the "()" prototype (which changes calling semantics, so beware ⚠️) or by thwarting the inlining mechanism some other way, like adding an explicit "return", as mentioned above:

sub not_inlined () { return 23 }

---

✨ Overriding Built-in Functions ✨

Many built-in functions may be overridden, though ngl this should only be attempted occasionally and for a genuinely good reason. Typically you'd do it when a package is trying to emulate missing built-in functionality on a non-Unix system. 🫶

Overriding can only be done by importing the name from a module at compile time — ordinary predeclaration just isn't good enough, sorry bestie. However, the "use subs" pragma lets you, in effect, predeclare subs through the import syntax, and those names may then override built-in ones:

use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }

To unambiguously refer to the built-in form, put the special package qualifier "CORE::" in front of the built-in name. For example, saying CORE::open() always refers to the built-in open(), even if the current package has imported some other subroutine called &open() from elsewhere. And even though it looks like a regular function call, it isn't — the "CORE::" prefix in that case is part of Perl's syntax, and works for any keyword, regardless of what's actually in the "CORE" package. Taking a reference to it, that is, "\&CORE::open", only works for some keywords. See CORE.

Library modules should not, in general, export built-in names like "open" or "chdir" as part of their default @EXPORT list, because these may sneak into someone else's namespace and change the semantics unexpectedly. 😭 Instead, if the module adds that name to @EXPORT_OK, then a user can import the name explicitly — just not implicitly. So they could say

use Module 'open';

and it would import the "open" override. But if they said

use Module;

they'd get the default imports without overrides.

The mechanism above for overriding built-ins is restricted — quite deliberately — to the package that requests the import. There's a second method that's sometimes applicable when you want to override a built-in everywhere, with zero regard for namespace boundaries. You achieve this by importing a sub into the special namespace "CORE::GLOBAL::". Here's an example that quite brazenly replaces the "glob" operator with something that understands regular expressions:

package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';

sub import {
    my $pkg = shift;
    return unless @_;
    my $sym = shift;
    my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
    $pkg->export($where, $sym, @_);
}

sub glob {
    my $pat = shift;
    my @got;
    if (opendir my $d, '.') {
        @got = grep /$pat/, readdir $d;
        closedir $d;
    }
    return @got;
}
1;

And here's how it could be (ab)used:

#use REGlob 'GLOBAL_glob';      # override glob() in ALL namespaces
package Foo;
use REGlob 'glob';              # override glob() in Foo:: only
print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules

The initial comment shows a contrived, even dangerous example. By overriding "glob" globally, you'd be forcing that new (and subversive) behavior onto the "glob" operator for every namespace, without the complete cognizance or cooperation of the modules that own those namespaces. Naturally this should be done with extreme caution — if it must be done at all. 🚨

The "REGlob" example above does not implement all the support needed to cleanly override Perl's "glob" operator. The built-in "glob" has different behaviors depending on whether it appears in scalar or list context, but our "REGlob" doesn't. And indeed, many Perl built-ins have those context-sensitive behaviors, and they have to be adequately supported by a properly written override. For a fully functional example of overriding "glob", go study the implementation of "File::DosGlob" in the standard library. 📚

When you override a built-in, your replacement should be consistent (if possible) with the built-in native syntax. You can achieve that by using a suitable prototype. To get the prototype of an overridable built-in, use the "prototype" function with an argument of "CORE::builtin_name" (see "prototype" in perlfunc).

Note however that some built-ins can't have their syntax expressed by a prototype at all (like "system" or "chomp"). If you override those, you won't be able to fully mimic their original syntax. 💀

The built-ins "do", "require" and "glob" can also be overridden, but due to special magic ✨ their original syntax is preserved, and you don't have to define a prototype for their replacements. (You can't override the "do BLOCK" syntax, though.)

"require" has special additional dark magic: if you invoke your "require" replacement as "require Foo::Bar", it will actually receive the argument "Foo/Bar.pm" in @_. See "require" in perlfunc.

And, as you'll have noticed from the previous example, if you override "glob", the "<*>" glob operator gets overridden too. 👑

In a similar fashion, overriding the "readline" function also overrides the equivalent I/O operator "<FILEHANDLE>". Also, overriding "readpipe" also overrides the operators ` and "qx//".

Finally, some built-ins (e.g. "exists" or "grep") can't be overridden. periodt.

---

🦋 Autoloading 🦋

If you call a subroutine that is undefined, you'd ordinarily get an immediate, fatal error complaining that the subroutine doesn't exist. (Same deal for subroutines being used as methods, when the method doesn't exist in any base class of the class's package.) However, if an "AUTOLOAD" subroutine is defined in the package or packages used to locate the original subroutine, then that "AUTOLOAD" subroutine gets called with the arguments that would have been passed to the original subroutine. The fully qualified name of the original subroutine magically appears in the global $AUTOLOAD variable of the same package as the "AUTOLOAD" routine. ✨ The name is not passed as an ordinary argument because, er, well, just because, that's why. (As an exception, a method call to a nonexistent "import" or "unimport" method is just skipped instead. Also, if the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the subroutine name. See "Autoloading with XSUBs" in perlguts for details.)

Many "AUTOLOAD" routines load in a definition for the requested subroutine using eval(), then execute that subroutine using a special form of goto() that erases the stack frame of the "AUTOLOAD" routine without a trace. 👻 (See the source to the standard module documented in AutoLoader, for example.) But an "AUTOLOAD" routine can also just emulate the routine and never define it. For example, let's pretend that a function that wasn't defined should just invoke "system" with those arguments. All you'd do is:

sub AUTOLOAD {
    our $AUTOLOAD;              # keep 'use strict' happy
    my $program = $AUTOLOAD;
    $program =~ s/.*:://;
    system($program, @_);
}
date();
who();
ls('-l');

Okay so real talk — if you predeclare the functions you wanna call that way, you don't even need the parentheses. Not me dropping a whole syntax glow-up:

    use subs qw(date who ls);
    date;
    who;
    ls '-l';

And if you want the full deluxe version of this whole vibe, go look at the Shell module on CPAN — it straight up treats undefined subroutine calls as calls to external programs. Main character behaviour, honestly. 💅

There's also a whole toolkit for module authors who wanna split their modules into autoloadable files. The lore:

• the standard AutoLoader module — described in AutoLoader and in AutoSplit 🎀
• the standard SelfLoader modules — described in SelfLoader 🦋
• the doc on adding C functions to Perl code — that's perlxs ✨

---

💖 Subroutine Attributes 💖

A subroutine declaration or definition can have a list of attributes riding along with it. If that attribute list is there, it gets broken up at space or colon boundaries and treated like a use attributes had happened. Wanna know which attributes are actually supported rn? Check attributes, bestie. And here's the slay part: unlike the limitation with the obsolescent use attrs, the sub : ATTRLIST syntax actually works for associating attributes with a pre-declaration, not just with a subroutine definition. Periodt. 👑

The attributes have to be valid as simple identifier names — so no punctuation allowed except the _ character. They can have a parameter list appended too, and that list is only checked for whether its parentheses ((, )) nest properly. That's it. That's the whole vibe check.

Examples of valid syntax (even though the attributes are unknown, which is iconic of them):

    sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
    sub plugh () : Ugly('\(") :Bad;
    sub xyzzy : _5x5 { ... }

Examples of invalid syntax, aka the ones that did NOT eat:

    sub fnord : switch(10,foo();    # ()-string not balanced
    sub snoid : Ugly('(');          # ()-string not balanced
    sub xyzzy : 5x5;                # "5x5" not a valid identifier
    sub plugh : Y2::north;          # "Y2::north" not a simple identifier
    sub snurt : foo + bar;          # "+" not a colon or space

The attribute list gets passed as a list of constant strings to the code that associates them with the subroutine. So like, that second valid example up there? Right now it actually looks like this in terms of how it's parsed and invoked:

    use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

For further details on attribute lists and how to manipulate them, see attributes and Attribute::Handlers. 🫶

---

🌈 SEE ALSO 🌈

• Wanna know more about references and closures? See "Function Templates" in perlref. 🔥
• Wanna learn about calling C subroutines from Perl? See perlxs.
• Wanna learn about calling Perl subroutines from C? See perlembed.
• Wanna learn about bundling up your functions in separate files? See perlmod.
• Wanna know what library modules come standard on your system? See perlmodlib.
• Wanna learn how to make object method calls? See perlootut.

---

And that's a wrap on the whole page, bestie!! 🎀✨ This was perl version v5.42.3, dated 2026-08-03, and it lives under the name PERLSUB(1). Go forth and sub, you absolute icon. 💜🦋💅