There's Java's collections as an example where interface List could represent an AbstractList, AbstractSequentialList, ArrayList, CopyOnWriteArrayList, LinkedList, RoleList, RoleUnresolvedList, Stack, Vector, or something else that implements what a list does, add(), and remove() and whatever else.

Tree::Binary uses Visitor_pattern to separate interface and implementation. You have Visitor::BreadFirstTraversal and Visitor::InOrderTraversal. Both are called with the visit method.

You could just pass in a sub reference to your interface containing the implementation.

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

sub multiply {
    my ( $x, $y, $implementation ) = @_;
    $implementation->( $x, $y );
}

sub whateverthedefaultis {
    my ( $x, $y ) = @_;
    return $x * $y;
}

# https://blogs.msdn.microsoft.com/matthew_van_eerde/2009/07/23/bad-pe
+rl-russian-peasant-multiplication-algorithm/

sub peasant {
    my ( $a, $b ) = @_;
    my $c;
    map { $c += $_ * $b }
      grep { $a & $_ }
      map { 1 << $_ } ( 0 .. log($a) / log 2 );
    return $c;
}

say multiply( 7, 3, \&whateverthedefaultis );
say multiply( 7, 3, \&peasant );
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Well I guess multiply should use the default.. by default if a third param is not passed in. Like how sort uses standard string comparison order by default. A whole lotta ways to do it.