It's been a while since I use Win32::SerialPort. When I first started using that module, Configuration and Capability Methods section of the documentation has information on timeouts that helped me get past my initial issues. In my case, I was trying to read too fast after sending a command. After reading that section, I made a few changes using that information helped me with my timing issue. Just sharing that in case it might help you.

Also, you might want to take a look at Control::CLI. It's a module to let you do telnet, SSH, and serial port stuff. It uses other modules underneath (including Win32::SerialPort on Windows) and the syntax is very similar to Win32::SerialPort. The reason that I'm recommending you to check it out is that has logging (input_log(), output_log(), dump_log()), which can help you debug what is being sent by your code and what it receives from the device. Also, it provides a debug() method that might help with your troubleshooting work.