traverse($in, 0, 0);

sub traverse {
  my( $ref, $reply_to, $depth) = @_;
  for (
    grep{ ($_->{dept} == $depth) && ($_->{r_to} == $reply_to) } @$ref
  ) {
    print ' ' x $_->{dept}, '-', $_->{comm}, "\n";
    traverse($ref, $_->{c_id}, ($depth + 1));
  }
}