package Tail;
use Moose;

sub wag { ... }

package Dog;
use Moose;

has tail => (
    is => 'ro',
    isa => 'Tail',
    default => sub { Tail->new },
    handles => { 'wag_tail' => 'wag' }
);

## later in your code ...

my $fido = Dog->new;
$fido->wag_tail; # is equivalent too $fido->tail->wag;