A couple of simple things if you had performance difficulties. After you have the $notifyObj you will notice that I don't actually use the Notify portion of Win32::ChangeNotify. You need to use the Tk timer event to check the file, you will see from the line:
So I call the notify object with a timeout of 0 - i.e. I ask it to return immediately with the status. Just to check, I have Benchmark already running in the application the code came from. Here are the timings for three runs through the changeNotofy:
Your solution does work with a minimal performance impact, but unfortunately it only gets triggered when the file is either opened or closed. The file I'm listening to is always open and getting updated once every x seconds depending on what the external app is doing.