Has-a relationship methods are generally not directly usable by callers of the delegating class. calling scratch directly on cat doesn't indicate which paw should scratch. Hence, the delegating class has-a method that uses it parameter(s) to determine which instance of paw should do the scratching.

#! perl -slw
use strict;

package paw;

sub new { my $class = shift; return bless { @_ }, $class; }
sub scratch { 
    my $self = shift;
    print "Scratching with $self->{ which } paw"; 
}

package cat;

sub new {
    my $class = shift;
    my %self = @_;
    $self{ paws } = { 
        map{ 
            $_ => paw->new( which => $_ )
        } 'left front', 'right front', 'left hind', 'right hind'
    };
    return bless \%self, $class;
}

sub scratchesWith{ 
    my( $self, $paw ) = @_;
    $self->{ paws }{$paw}->scratch();
}

sub speaks { 
    my $self = shift;
    print $self->{name}, ' says: ',  $self->{voice}; 
}

1;

package main;

my $cat = cat->new( 
    name => 'Tiddles',
    voice=> 'Meow!',
);

$cat->scratchesWith( 'right front' );
$cat->speaks();

__END__
[ 4:08:26.92] P:\test>405381
Scratching with right front paw
Tiddles says: Meow!
[download]