#!/usr/bin/env perl

package PairUnique;

use strict;
use warnings;

sub new {return bless {}, shift}

sub unique {
    my ($self, $k, $v) = @_;
    return !$self->{"$k=>$v"}++ ? $v : ();
}

package main;

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

my %hash;
{
    my $get = PairUnique->new;
    while (<DATA>) {
        chomp;
        my ($k, $v) = split /\s/;

        my $aref = $hash{$k} //= [];

        push @$aref, $get->unique($k,$v);
    }
}

print Dumper \%hash;

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