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

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

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

%category = (
  1 => [ 'One',      [5, 6, 7] ],
...
  7 => [ 'Eight',    [] ],
  ...
);

  sub print_categories {
    my $level = shift;
...
  }

  print_categories(0, @top_level_categories);