Variables in Perl 6

Variable names start with a special character called a sigil, followed optionally by a second special character named twigil and then an identifier.


The sigil serves both as a rough type constraint and as an indicator as to whether the contents of the variable flatten in list context. See also the documentation in List.

Sigil Type constraint Default type Flattens Assignment
$ Mu (no type constraint) Any No item
& Callable Callable No item
@ Positional Array Yes list
% Associative Hash Yes list


my $square = 9 ** 2;
my @array  = 1, 2, 3;   # Array variable with three elements
my %hash   = London => 'UK', Berlin => 'Germany';

Variables without sigils are also possible, see sigilless variables.

Item and List Assignment

There are two types of assignment, item assignment and list assignment. Both use the equal sign = as operator. The distinction whether an = means item or list assignment is based on the syntax of the left-hand side. (TODO: explain in detail, or do that in operators).

Item assignment places the value from the right-hand side into the variable (container) on the left.

List assignment leaves the choice of what to do to the variable on the left.

For example, Array variables (@ sigil) empty themselves on list assignment and then put all the values from the right-hand side into themselves. Contrary to item assignment, it means that the type of the variable on the left always stays Array, regardless of the type of the right-hand side.

Note that item assignment has tighter precedence than list assignment and also tighter than the comma. Thus:

my @array = my $num = 42, "str";

assigns 42 to $num, and both 42 and "str" to @array. See operators for more details.

Sigilless variables

It is possible to create "variables" in Perl 6 that do not have sigils:

my \degrees = pi / 180;
my \θ       = 15 * degrees;

Note, however, that these do not create containers. That means degrees and θ above actually directly represent Nums. To illustrate, try assigning to one after you've defined it:

θ = 3; # Dies with the error "Cannot modify an immutable Num"


Twigils influence the scoping of a variable. Please be aware that twigils have no influence over whether the primary sigil interpolates. That is, if $a interpolates, so do $^a, $*a, $=a, $?a, $.a, etc. It only depends on the $.

Twigil Scope
* dynamic
! attribute (class member)
? compile-time variable
. method (not really a variable)
< index into match object (not really a variable)
^ self-declared formal positional parameter
: self-declared formal named parameter
= Pod variables
~ the sublanguage seen by the parser at this lexical spot

The * Twigil

Dynamic variables are looked up through the caller, not through the outer scope. For example:

my $lexical   = 1;
my $*dynamic1 = 10;
my $*dynamic2 = 100;

sub say-all() {
    say "$lexical, $*dynamic1, $*dynamic2";

# prints 1, 10, 100

    my $lexical   = 2;
    my $*dynamic1 = 11;
    $*dynamic2    = 101;

    # prints 1, 11, 101

# prints 1, 10, 101

The first time &say-all is called, it prints "1, 10, 100" just as one would expect. The second time though, it prints "1, 11, 101". This is because $lexical isn't looked up in the caller's scope but in the scope &say-all was defined in. The two dynamic variables are looked up in the caller's scope and therefore have the values 11 and 101. The third time &say-all is called $*dynamic1 isn't 11 anymore, but $*dynamic2 is still 101. This stems from the fact that we declared a new dynamic variable $*dynamic1 in the block and did not assign to the old variable as we did with $*dynamic2.

The ! Twigil

Attributes are variables that exist per instance of a class. They may be directly accessed from within the class via !:

class Point {
    has $.x;
    has $.y;

    method Str() {
        "($!x, $!y)"

Note how the attributes are declared as $.x and $.y but are still accessed via $!x and $!y. This is because in Perl 6 all attributes are private and can be directly accessed within the class by using $!attribute-name. Perl 6 may automatically generate accessor methods for you though. For more details on objects, classes and their attributes see objects.

The ? Twigil

Compile-time variables may be addressed via the ? twigil. They are known to the compiler and may not be modified after being compiled in. A popular example for this is:

say "$?FILE: $?LINE"; # prints " 23" if this is the 23 line of a
                      # file named "".

Although they may not be changed at runtime, the user is allowed to (re)define such variables.

constant $?TABSTOP = 4; # this causes leading tabs in a heredoc or in a POD
                        # block's virtual margin to be counted as 4 spaces.

For a list of these special variables see Compile-time variables.

The . Twigil

The . twigil isn't really for variables at all. In fact, something along the lines of

class Point {
    has $.x;
    has $.y;

    method Str() {
        "($.x, $.y)" # note that we use the . instead of ! this time

just calls the methods x and y on self, which are automatically generated for you because you used the . twigil when the attributes were declared. Note, however, that subclasses may override those methods. If you don't want this to happen, use $!x and $!y instead.

The fact that the . twigil just does a method call also implies that the following is possible too:

class SaySomething {
    method a() { say "a"; }
    method b() { $.a; }

SaySomething.a; # prints "a"

For more details on objects, classes and their attributes and methods see objects.

The < Twigil

The < twigil is just an alias for $/<...> where $/ is the match variable. For more information about the match variable see $/ and type Match.

The ^ Twigil

The ^ twigil declares a formal positional parameter to blocks or subroutines. Variables of the form $^variable are a type of placeholder variable. They may be used in bare blocks to declare formal parameters to that block. So the block in the code

    for ^4 {
        say "$^second follows $^first";

which prints

1 follows 0
3 follows 2

has two formal parameters, namely $first and $second. Note that even though $^second appears before $^first in the code, $^first is still the first formal parameter to that block. This is because the placeholder variables are sorted in Unicode order. If you have self-declared a parameter using $^a once, you may refer to it using only $a thereafter.

Subroutines may also make use of placeholder variables but only if they do not have an explicit parameter list. This is true for normal blocks too.

sub say-it    { say $^a; } # valid
sub say-it()  { say $^a; } # invalid
              { say $^a; } # valid
-> $x, $y, $x { say $^a; } # invalid

Placeholder variables syntactically cannot have any type constraints. Be also aware that one cannot have placeholder variables with a single upper-case letter. This is disallowed in favor of being to able to catch some Perl 5-isms.

The : Twigil

The : twigil declares a formal named parameter to a block or subroutine. Variables declared using this form are a type of placeholder variable too. Therefore the same things that apply to variables declared using the ^ twigil also apply here (with the exception that they are not positional and therefore not ordered using Unicode order, of course). So this:

    say { $:add ?? $^a + $^b !! $^a - $^b }( 4, 5 ) :!add

Will print "-1".

See ^ for more details about placeholder variables.

The = Twigil

The = twigil is used to access Pod variables. Every Pod block in the current file can be accessed via a Pod object, such as $=data, $=SYNOPSIS or =UserBlock. That is: a variable with the same name of the desired block and a = twigil.

=begin Foo
=end Foo

#after that, $=Foo gives you all Foo-Pod-blocks

You may access the Pod tree which contains all Pod structures as a hierarchical data structure through $=pod.

Note that all those $=someBlockName support the Positional and the Associative roles.

The ~ Twigil

Note: Slangs are NYI in Rakudo.

The ~ twigil is for referring to sublanguages (called slangs). The following are useful:

$~MAIN       the current main language (e.g. Perl statements)
$~Quote      the current root of quoting language
$~Quasi      the current root of quasiquoting language
$~Regex      the current root of regex language
$~Trans      the current root of transliteration language
$~P5Regex    the current root of the Perl 5 regex language

You may supersede or augment these languages in your current lexical scope by using

augment slang Regex {  # derive from $~Regex and then modify $~Regex
    token backslash:std<\Y> { YY };


supersede slang Regex { # completely substitute $~Regex

Variable Declarators and Scope

Most of the time it's enough to create a new variable using the my keyword:

    my $amazing-variable = "World";
    say "Hello $amazing-variable!"; # Hello World!

However, there are many declarators that change the details of scoping beyond what Twigils can do.

Declarator Effect
my Introduces lexically scoped names
our Introduces package-scoped names
has Introduces attribute names
anon Introduces names that are private to the construct
state Introduces lexically scoped but persistent names
augment Adds definitions to an existing name
supersede Replaces definitions of an existing name

There are also two prefixes that resemble declarators but act on predefined variables:

Prefix Effect
temp Restores a variable's value at the end of scope
let Restores a variable's value at the end of scope if the block exits unsuccessfully

The my Declarator

Declaring a variable with my gives it lexical scope. This means it only exists within the current block. For example:

    my $foo = "bar";
    say $foo; # -> "bar"
say $foo; # !!! "Variable '$foo' is not declared"

This dies because $foo is only defined as long as we are in the same scope.

Additionally, lexical scoping means that variables can be temporarily redefined in a new scope:

my $location = "outside";

sub outer-location {
    # Not redefined:
    say $location;

outer-location; # -> "outside"

sub in-building {
    my $location = "inside";
    say $location;

in-building; # -> "inside"

outer-location; # -> "outside"

If a variable has been redefined, any code that referenced the outer variable will continue to reference the outer variable. So here, &outer-location still prints the outer $location:

sub new-location {
    my $location = "nowhere"

new-location; # -> "outside"

To make new-location() print nowhere, make $location a dynamic variable using the * twigil.

The our Declarator

our variables work just like my variables, except that they also introduce an alias into the symbol table.

module M {
    our $Var;
    # $Var available here

# Available as $M::Var here.

The has Declarator

The anon Declarator

The state Declarator

The augment Declarator

The supersede Declarator

The temp Prefix

Like my, temp restores the old value of a variable at the end of its scope. However, temp does not create a new variable.

my $cookies = 5;

sub buy-cookie { $cookies++ }

    my $cookies = 42;
    buy-cookie;   # Increments the outer $cookies variable
    say $cookies; # 42
                  # the outer $cookies variable is 6 now

    temp $cookies = 42;
    # Still the same $cookies, but a new value:
    say $cookies; # -> 43

# Old value is restored
say $cookies; # -> 6

The let Prefix

Like the temp listop, but only restores the value if the block exits unsuccessfully.

Type Constraints and Initialization

Variables can have a type constraint, which goes between the declarator and the variable name:

    my Int $x = 42;
    $x = 'a string';    # throws an X::TypeCheck::Assignment error

If a scalar variable has a type constraint but no initial value, it is initialized with the type object of the constraint type.

my Int $x;
say $x.^name;       # Int
say $x.defined;     # False

Scalar variables without an explicit type constraint are typed as Mu but default to the Any type object.

Variables with the @ sigil are initialized with an empty Array; variables with the % sigil are initialized with an empty Hash.

Special Variables

Perl 5 is infamous for its many obscure special variables. Perl 6 also has special variables but only has three that are extra short due to how often they're used. Other special variables have longer, more descriptive names.

Pre-defined lexical variables

There are three special variables that are available in every block:

Variable Meaning
$_ topic variable
$/ regex match
$! exceptions

The $_ Variable

$_ is the topic variable. It is the default parameter for blocks that do not have an explicit signature, so constructs like for @array { ... } and given $var { ... } bind to $_ simply by invoking the block.

for <a b c> { say $_ }  # sets $_ to 'a', 'b' and 'c' in turn
say $_ for <a b c>;     # same, even though it's not a block
given 'a'   { say $_ }  # sets $_ to 'a'
say $_ given 'a';       # same, even though it's not a block

CATCH blocks set $_ to the exception that was caught. The ~~ smart-match operator sets $_ on the right-hand side expression to the value of the left-hand side.

Calling a method on $_ can be shortened by leaving off the variable name:

.say;                   # same as $_.say

m/regex/ and /regex/ regex matches and s/regex/subst/ substitutions work on $_:

say "Looking for strings with non-alphabetic characters...";
for <ab:c d$e fgh ij*> {
    .say if m/<!alpha>/;

This outputs:

Looking for strings with non-alphabetic characters...

The $/ Variable

$/ is the match variable. It stores the result of the last Regex match and so usually contains objects of type Match.

'abc 12' ~~ /\w+/;  # sets $/ to a Match object
say $/.Str;         # abc

The Grammar.parse method also sets the caller's $/ to the resulting Match object.

use XML;
XML.parse(slurp "filename.xml")
say $/;

Positional Attributes

$/ can have positional attributes if the Regex had capture-groups in it, which are just formed with parentheses.

'abbbbbcdddddeffg' ~~ / a (b+) c (d+ef+) g /;
say $/[0]; # 「bbbbb」
say $/[1]; # 「dddddeff」

These can also be accessed by the shortcuts $0, $1, $2, etc.

say $0; # 「bbbbb」
say $1; # 「dddddeff」

To get all of the positional attributes, any of $/.list, @$/, or @() can be used.

say @().join; # bbbbbdddddeff

Named Attributes

$/ can have named attributes if the Regex had named capture-groups in it, or if the Regex called out to another Regex.

'I.... see?' ~~ / \w+ $<punctuation>=[ <-[\w\s]>+ ] \s* $<final-word> = [ \w+ . ] /;
say $/<punctuation>; # 「....」
say $/<final-word>;  # 「see?」

These can also be accessed by the shortcut $<named>.

say $<punctuation>; # 「....」
say $<final-word>;  # 「see?」

To get all of the named attributes, any of $/.hash, %$/, or %() can be used.

say %().join;       # "punctuation  see?"

The $! Variable

$! is the error variable. If a try block or statement prefix catches an exception, that exception is stored in $!. If no exception was caught, $! is set to the Any type object.

Note that CATCH blocks do not set $!. Rather they set $_ inside the block to the caught exception.

Compile-time variables

$?FILE Which file am I in?
$?LINE Which line am I at?
&?ROUTINE Which routine am I in?
&?BLOCK Which block am I in?
%?LANG What is the current set of interwoven languages?

Other compile-time variables:

$?SCOPE Which lexical scope am I in?
$?PACKAGE Which package am I in?
$?MODULE Which module am I in?
$?CLASS Which class am I in? (as variable)
$?ROLE Which role am I in? (as variable)
$?GRAMMAR Which grammar am I in?
$?TABSTOP How many spaces is a tab in a heredoc or virtual margin?
$?USAGE The usage message generated from the signatures of MAIN subs.
$?ENC Default encoding of Str.encode/Buf.decode/various IO methods.

Dynamic variables

current working directory $*CWD

$*ARGFILES Magic command-line input handle.
@*ARGS Arguments from the command line.
$*IN Standard input filehandle.
$*OUT Standard output filehandle.
$*ERR Standard error filehandle.
@*INC Each repository is represented as an object with the CompUnitRepo interface in the @*INC array
$*TZ The system's local timezone.
$*CWD The Current Working Directory.
$*KERNEL Which kernel am I compiled for?
$*DISTRO Which OS distribution am I compiling under?
$*VM Which virtual machine am I compiling under?
$*PERL Which Perl am I compiled for?
$*PID Process ID of the current process.
$*PROGRAM_NAME Path to the current executable as it was entered on the command line, or C<-e> if perl was invoked with the -e flag.
$*EXECUTABLE Absolute path of the perl executable that is currently running.
$*EXECUTABLE_NAME The name of the perl executable that is currently running. (e.g. perl6-p, perl6-m, Niecza.exe) Favor $*EXECUTABLE because it is not guaranteed that the perl executable is in PATH.

This documentation was generated from variables.pod.