Basics

That feature is built into Raku (formerly known as Perl 6). Here is the equivalent of your Getopt::Long code in Raku:

sub MAIN ( Str  :$file    = "file.dat"
         , Num  :$length  = Num(24)
         , Bool :$verbose = False
         )
{
    $file.say;
    $length.say;
    $verbose.say;
}

MAIN is a special subroutine that automatically parses command line arguments based on its signature.

Str and Num provide string and numeric type constraints.

Bool makes $verbose a binary flag which is False if absent or if called as --/verbose. (The / in --/foo is a common Unix command line syntax for setting an argument to False).

: prepended to the variables in the subroutine signature makes them named (instead of positional) parameters.

Defaults are provided using $variable = followed by the default value.

Aliases

If you want single character or other aliases, you can use the :f(:$foo) syntax.

sub MAIN ( Str  :f(:$file)    = "file.dat"
         , Num  :l(:$length)  = Num(24)
         , Bool :v(:$verbose) = False
         )
{
    $file.say;
    $length.say;
    $verbose.say;
}

:x(:$smth) makes additional alias for --smth such as short alias -x in this example. Multiple aliases and fully-named is available too, here is an example: :foo(:x(:bar(:y(:$baz)))) will get you --foo, -x, --bar, -y and --baz and if any of them will pass to $baz.

Positional arguments (and example)

MAIN can also be used with positional arguments. For example, here is Guess the number (from Rosetta Code). It defaults to a min of 0 and max of 100, but any min and max number could be entered. Using is copy allows the parameter to be changed within the subroutine:

#!/bin/env perl6
multi MAIN
#= Guessing game (defaults: min=0 and max=100)
{
    MAIN(0, 100)
}

multi MAIN ( $max )
#= Guessing game (min defaults to 0)
{
    MAIN(0, $max)
}

multi MAIN
#= Guessing game
( $min is copy #= minimum of range of numbers to guess
, $max is copy #= maximum of range of numbers to guess
)
{
    #swap min and max if min is lower
    if $min > $max { ($min, $max) = ($max, $min) }

    say "Think of a number between $min and $max and I'll guess it!";
    while $min <= $max {
        my $guess = (($max + $min)/2).floor;
        given lc prompt "My guess is $guess. Is your number higher, lower or equal (or quit)? (h/l/e/q)" {
            when /^e/ { say "I knew it!"; exit }
            when /^h/ { $min = $guess + 1      }
            when /^l/ { $max = $guess          }
            when /^q/ { say "quiting"; exit    }
            default   { say "WHAT!?!?!"        }
        }
    }
    say "How can your number be both higher and lower than $max?!?!?";
}

Usage message

Also, if your command line arguments don't match a MAIN signature, you get a useful usage message, by default. Notice how subroutine and parameter comments starting with #= are smartly incorporated into this usage message:

./guess --help
Usage:
  ./guess -- Guessing game (defaults: min=0 and max=100)
  ./guess <max> -- Guessing game (min defaults to 0)
  ./guess <min> <max> -- Guessing game

    <min>    minimum of range of numbers to guess
    <max>    maximum of range of numbers to guess

Here --help isn't a defined command line parameter, thus triggering this usage message.

See also

See also the 2010, 2014, and 2018 Perl 6 advent calendar posts on MAIN, the post Parsing command line arguments in Perl 6, and the section of Synopsis 6 about MAIN.