Davorg's post above is on point, but here is how I deal with datastructures like this. Basically I localize a hash ref, to make the code slightly more visually appealing, ala..

for ( keys %category_hash ) {
   my $first_lvl = $category_hash{$_};
   print "$_\n";

   for ( keys %{ $first_lvl } ) {
      my $second_lvl = $first_lvl->{$_};
      print ' ' x 3 . "$_"\n";

      for ( keys %{ $second_lvl } ) {
         print ' ' x 6 . "$_ : $second_lvl->{$_}\n";
      }
   } # END for keys $first_lvl
}    # END for keys %category_hash
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Grabbing a ref to the hash will be slightly better than creating a copy of the sub hashes as you go along. Personally I think it might be better to define a better structure like recommended above.

Regards

MMMMM... Chocolaty Perl Goodness.....