use strict;
my %cat;
$cat{'bird',0} = 'bluejay';
$cat{'bird',1}= 'oriole';
$cat{'mammal',0} = 'cat';
$cat{'mammal',1} = 'dog';

for (sort keys %cat) {
  my ($class, $num) = split $;;
  print "cat[$class][$num] = $cat{$_}\n";
}