use Devel::Symdump;
sub make_animal {
    my $sound = shift;
    my @animals_that_make_sound 
        = grep { eval { $_->isa( 'Animal' ) && $_->makes_sound eq $sound } }
            Devel::Symdump->rnew->packages;
    die "more than one animal can $sound\n" if @animals_that_make_sound > 1;
    die "no animals $sound\n" unless @animals_that_make_sound;
    return $animals_that_make_sound[0]->new;
}

print make_animal( 'oink' );