$selector = IO::Select->new();
  $selector->add(*CMD_ERR, *CMD_OUT);

  while(@ready = $selector->can_read) {
    foreach my $fh (@ready) {
      if(fileno($fh) == fileno(CMD_ERR)) {
        my $line = scalar <CMD_ERR>;
        print $line;
      }
    }
  }