SELECT category_id, category_name, main_category
  FROM test
  ORDER BY main_category, category_id

##</code><code>##

  $category{$category_id} = [ $category_name, [] ];

##</code><code>##

push @{$category{$main_category}->[1]}, $category_id;

##</code><code>##

%category = (
  1 => [ 'One',      [5, 6, 7] ],
  2 => [ 'Two',      [8, 9] ],
  3 => [ 'Three',    [] ],
  4 => [ 'Four',     [] ],
  5 => [ 'Six',      [10, 13] ],
  6 => [ 'Seven',    [] ],
  7 => [ 'Eight',    [] ],
  ...
);

##</code><code>##

  sub print_categories {
    my $level = shift;
    foreach my $id (@_) {
      print "$level: $category{$id}->[0]\n";
      print_categories($level + 1, @{$category{$id}->[0]});
    }
  }

  print_categories(0, @top_level_categories);