traverse($in,0,0, sub {print ' ' x $_->{dept},'-',$_->{comm},"\n";});

sub traverse {
  my( $ref, $reply_to, $depth, $code) = @_;
  for my $node (
    grep{ ($_->{dept} == $depth) && ($_->{r_to} == $reply_to) } @$ref
  ) {
    local $_ = $node;
    $code->();
    traverse($ref, $node->{c_id}, ($depth + 1), $code);
  }
}