Try this out. The value of the hash is a reference to an array. You can push a value onto that array just like a simple @array.

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %descriptions;

while (<DATA>)
{
   my ($animal,$hyper,$description) = (split)[0,2,3];
   if ($hyper eq 'hyper')
   {
      push @{$descriptions{$animal}}, $description;
   }
}
print Dumper  \%descriptions;

=prints
$VAR1 = {
          'alligator-n' => [
                             'animal-n',
                             'beast-n',
                             'carnivore-n'
                           ]
        };
=cut        

__DATA__
alligator-n amphibian_reptile   event   sleep-v
alligator-n amphibian_reptile   event   swim-v
alligator-n amphibian_reptile   event   walk-v
alligator-n amphibian_reptile   hyper   animal-n
alligator-n amphibian_reptile   hyper   beast-n
alligator-n amphibian_reptile   hyper   carnivore-n
alligator-n amphibian_reptile   mero    foot-n
alligator-n amphibian_reptile   mero    jaw-n
[download]