I've never used this module, but i'm going to make some guesses/suggestions based on intuition...

1. You may have a permissions problem, check what the file permissions are for /dev/ttyS0.
2. You should follow the examples from the perldocs more closely, it says that you should check new for a true return value, otherwise something may have gone wrong, change it to...

   $port = new Device::SerialPort ( "/dev/ttyS0") 
         or die "can't open port: $!";
   [download]

3. I don't know of any perl implimentation that supports a "-W" option, i think you ment to use "-w."