my $in = [
  {c_id => 1,  r_to => 0, dept => 0, p_id => 1, comm => 'this',},
  {c_id => 3,  r_to => 1, dept => 1, p_id => 1, comm => 'the other',},
  {c_id => 5,  r_to => 1, dept => 1, p_id => 1, comm => 'else',},
  {c_id => 6,  r_to => 5, dept => 2, p_id => 1, comm => 'that',},
  {c_id => 7,  r_to => 6, dept => 3, p_id => 1, comm => 'moreover',},
  {c_id => 9,  r_to => 3, dept => 2, p_id => 1, comm => 'no way',},
  {c_id => 10, r_to => 9, dept => 3, p_id => 1, comm => 'give',}, 
];

##</code><code>##

1: this
  - 3: the other
    - 9: no way
      - 10: give
  - 5: else
    - 6: that
      - 7: moreover

##</code><code>##

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);
  }
}

##</code><code>##

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));
  }
}