"Composition_over_inheritance" was the mantra that i learned back in 1998 when i attended my first advanced OO class in college. The bad news is that it took me nearly 10 years to understand. :) The good news is that packages like Moose make all of this easier to implement than not. Need to define an interface?

package My::Abstract::Interface;
use Moose::Role;
requires qw( foo bar baz qux );
[download]

package My::Abstract::Interface;
use Carp;

sub foo { croak }
sub bar { croak }
sub baz { croak }
sub qux { croak }
[download]

#!/usr/bin/env perl 

package Foo;
use Moose;
has string => ( is => 'rw', isa => 'Str' );
sub to_string { shift->string }

package Bar;
use Moose;
has _foo => ( is => 'rw', isa => 'Foo', handles => ['string'] );
around BUILDARGS => sub {my($o,$c)=(shift,shift);$c->$o(_foo => Foo->n
+ew(@_))};
sub to_string { uc shift->string }

package main;
my $foo = Foo->new( string => 'hello world' );
print $foo->to_string, $/;

my $bar = Bar->new( %$foo );
print $bar->to_string, $/;
[download]

UPDATE:
Changed arguments to Bar constructor. (Was the same as arguments to Foo constructor.)

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)