Perl 6 Deep Dive

Parameter placeholders

Even if the signature of the sub is missing, the sub  can still take and use parameters. Perl 6 defines the so-called placeholders, which are the variables with the ^ twigil inside a sub . We can see this in the following code: 

sub greet {
    say "Hello, $^name!";
}

greet('Mark'); # Hello, Mark!

In this code, the $^name variable takes the value of the string passed at the function call. The value becomes a read-only parameter of the sub.

If there is more than one parameter, their order corresponds to the alphabetical order of the placeholders:

sub subtract {
    $^b - $^a
}
say subtract(10, 8); # -2

The values 10 and 8 reside in the $^a and $^b variables here.

When using placeholders, a function cannot have a signature that ...