if ($PortObj->can_wait_modemlines) {
      $rc = $PortObj->wait_modemlines( MS_RLSD_ON );
      if (!$rc) { print "carrier detect changed\n"; }
  }

  if ($PortObj->can_modemlines) {
     $ModemStatus = $PortObj->modemlines;
     if ($ModemStatus & $PortObj->MS_RLSD_ON) { print "carrier detected\n"; }
  }