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 );
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. | 
