Paco's solution is probably what you want. man perlreftut or see Understand References Today for more info on the subject.

But there's always the good old-fashioned Perl 4 approach:

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";
}
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For sparse arrays whose use will be dominated by looking up things by known indices rather than processing rows or columns in a loop, I sometimes still use this. Note that it's inappropriate if your keys might contain binary data, because they can't contain the value of $;, by default \034.