This uses a hash-of arrays:

use warnings;
use strict;
use Data::Dumper;
$Data::Dumper::Sortkeys=1;

my %animals;
while (<DATA>) {
    my @cols = split;
    next unless $cols[2] eq 'hyper';
    push @{ $animals{$cols[0]} }, $cols[3];
}
print Dumper(\%animals);

__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]

prints:

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

See also:

• perldoc perldsc