You shouldn't have the %animal_sounds as a global variable in the main namespace (main:: or ::), but maybe have it as a global variable in the Animal:: namespace. You don't need the BEGIN blocks around your animal registration section, and you also don't break stricture.

Personally, I would use any of the Pluggable modules, like Module::Pluggable, especially when you want new plugins to become automatically discovered when they are available:

package Animal;
use Carp qw(croak);
use Module::Pluggable 
    require => 1,
    search_path => 'Animal', 
    sub_name => known_animals;

# use Memoize;
# memoize('known_animals');

my @known_animals = __PACKAGE__->known_animals;
my %animal_sounds;

for my $animal (@known_animals) {
    for my $sound ($animal->sounds) {
        croak "Ambigous sound '$sound' ($animal / $animal_sounds{$soun
+d})"
            if exists $animal_sounds{$sound};
        $animal_sounds{$sound} = $animal;
    };
};

# and in a separate file Animal/Dog.pm:
package Animal::Dog;
sub sounds { 'woof' };
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