Hi thanks for the reply, I thought of it so but then I view the codes and could not find any possibility that they could have cause the hang up. Below are the codes, would appreciate if you all help me take a look.

sub readXML{
    # create object
    $xml = new XML::Simple (KeyAttr=>[]);

    # read XML file
    $data = $xml->XMLin($config_path  . "sensors.xml");
}

sub writeValueXML{
    while(! (-x $config_path . "sensors.xml")){}     #wait until other
+ process finish writing the file
    system("sudo chmod 644 $config_path" . "sensors.xml");    #set per
+mission for root to rw only (exclusive write)

      my $output = new IO::File(">" . $config_path . "sensors.xml");
      my $writer = new XML::Writer(OUTPUT => $output);
    my $flag = 0;;
    $count = 0;

      $writer->startTag("Sensors");
    while($count < $DATANUM){
        #write xml values
          $e = $data->{$sensor_type[$count]};
        
        $writer->startTag("$sensor_type[$count]");
        $writer->startTag("value");
        $writer->characters($vars[$count]);            
         $writer->endTag("value");
        $writer->startTag("threshold");
        $writer->characters($e->{threshold});            
         $writer->endTag("threshold");
        $writer->endTag("$sensor_type[$count]");
        
        if(($vars[$count]*1.0) > ($e->{threshold}*1.0)){    #*1 to for
+ce it to float type
            if (-e $config_path . "SensorAlert.txt" && !$flag){
                while(! (-x $config_path . "SensorAlert.txt")){}     #
+wait until other process finish writing the file
                unlink($config_path . "SensorAlert.txt");    #set perm
+ission for root to rw only (exclusive write)
            }

            open WRITEFILE, "+>>", $config_path . "SensorAlert.txt";
            print WRITEFILE "$count:$vars[$count]\n";
            close(WRITEFILE);

            $flag++;
        }
        $count++;
    }

    $e = $data->{Weather_Data_Interval};
    $writer->startTag("Weather_Data_Interval");
    $writer->startTag("value");
    $writer->characters($e->{value});            
     $writer->endTag("value");    
    $writer->endTag("Weather_Data_Interval");

     $writer->endTag("Sensors");
      $writer->end();

      $output->close();

    if($flag){
        read_config_file;
        if ($config_data[10] ne "Sensors:disabled\n"){
            $config_data[10] = "Sensors:true\n";
            write_config_file;
        }
        system("sudo chmod 777 $config_path" . "SensorAlert.txt");
    }
    system("sudo chmod 777 $config_path" . "sensors.xml");
}

sub appendLoggedFile{
    my ($second, $minute, $hour, $dayOfMonth, $month, $yearOffset, $da
+yOfWeek, $dayOfYear, $daylightSavings) = localtime();
    $year = 1900 + $yearOffset;
    my $theTime = "$hour:$minute:$second, $weekDays[$dayOfWeek] $month
+s[$month] $dayOfMonth, $year";

      open WRITEFILE, "+>>", $data_path . "$months[$month]_$dayOfMonth
+" . "_SensorLog.txt" or die $!;
    print WRITEFILE $values . ",\t\t" . $theTime . "\n";
    close(WRITEFILE);
}
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and roboticus, when the program hang, I end the process and use minicom to read from the port again, and the data is still transmittin. I cannot use 2 programs to read the same port at the same time, as it will say the port is being used by other process right now.

Thanks!