#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my %hash;

while (<DATA>) {
    chomp;
    my ($k, $v) = split /\s/;
    $hash{$k}{$v} = undef;
}

$hash{$_} = [keys %{$hash{$_}}] for keys %hash;

print Dumper \%hash;

__DATA__
bird beak
bird beak
bird claw
bird wings
bird feathers
snake fangs
snake scales
snake fangs
snake tail