my @rows = ({id=>0,fur=>'t',legs=>'2',tail=>'y'},
            {id=>1,fur=>'t',legs=>'2',tail=>'y'},
            {id=>2,fur=>'t',legs=>'2',tail=>'y'},
            {id=>3,fur=>'f',legs=>'4',tail=>'y'},
            {id=>4,fur=>'f',legs=>'4',tail=>'y'},
         );

my %lookup_id = map { $_->{id}, $_ } @rows;

my %lookup_fur_and_legs;
while (my ($id,$ref) = each %lookup_id) {
  my $key = $ref->{fur} . $ref->{legs};
  if (exists $lookup_fur_and_legs{$key}) {
    push  @{$lookup_fur_and_legs{$key}}, $ref;
  } else {
    $lookup_fur_and_legs{$key} = [$ref];
  }
}