{   package Animal;
    use Carp qw( croak );    
    sub new { bless \my $scalar, shift };
}
{   package Sheep; use base 'Animal'; sub makes_sound { "baa"  } }
{   package Cow;   use base 'Animal'; sub makes_sound { "moo"  } }
{   package Pig;   use base 'Animal'; sub makes_sound { "oink" } }

##</code><code>##

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' );