use strict;
use warnings;

sub print_tree {
   my ($tree, $depth) = @_;
   $depth ||= 0;
   my $indent = '...' x $depth;
   for (sort keys %$tree) {
      print($indent, /^\s*\z/ ? "<blank>" : $_, "\n");
      print_tree($tree->{$_}, $depth+1);
   }      
}

my %categories = (
    level1 => {
        'level1-1' => {},
        'level1-2' => {'level1-2-1' => {}},
        'level1-3' => {},
        },
    level2 => {'level2-1' => {}},
    );


print_tree(\%categories);

##</code><code>##

level1
...level1-1
...level1-2
......level1-2-1
...level1-3
level2
...level2-1