I have use both Win32::GUI and Tk. I found that I picked up the widget positioning in Win32::GUI far quicker that with Tk. Although I have since spent some time creating a Tk interface and the plotting seems far more thourgh in Tk..

I also believe that by using the Tk library you will be able to produce code that will be able to run on both MS windows as well as X-Windows. I have not heard of anyone even trying to use Win32::GUI on anything other than MS Windows.

So in a nutshell I would use Tk .. it may take a little longer to grasp (esp. if you're like me that is incredibly slow to pick up anything new) but I think you will get far more milage out of your code in terms of portability if you use Tk.

Update: Jeesh I battled with the english in that last sentence.