It's a little tricky, but not hard once you've seen it done once:

  my %hash;
  my $value = "foo";
  my @cats = qw(a b c d);

  my $p = \%hash;
  foreach my $item (@cats) {
      $p->{$item} ||= {};
      $p = $p->{$item};
  }
  $p->{_val} = $value;

  use Data::Dumper;
  print Dumper(\%hash);
[download]

Output:

$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => {
                                     'd' => {
                                              '_val' => 'foo'
                                            }
                                   }
                          }
                 }
        };
[download]

The trick here is to keep a pointer ($p) to the insertion-point for the next category as you walk through the structure. You also need some way to distinguish values from categories - in this case I used a special "_val" key.

-sam