#!/usr/bin/perl

# https://perlmonks.org/?node_id=1222551

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

my %hash;

while( <DATA> )
  {
  /(\S+)\s+(\S+)/ and $hash{$1} = [ keys %{{map {$_, 1} @{$hash{$1}}, $2}} ];
  }

print Dumper \%hash;

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