#!/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

##</code><code>##

$VAR1 = {
          'snake' => [
                       'tail',
                       'fangs',
                       'scales'
                     ],
          'bird' => [
                      'claw',
                      'wings',
                      'beak',
                      'feathers'
                    ]
        };

##</code><code>##

my %hash;

{
    my %attribseen;
    while (<DATA>) {
        chomp;
        my ($k, $v) = split /\s/;
        push @{$hash{$k}}, $v unless $attribseen{"$k|$v"}++;
    }
}

print Dumper \%hash;

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

##</code><code>##

#!/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