I think now is a good time to learn about Windows and how window handles work in Windows. This topic is not Perl specific.

You can look at the output and code of spy.pl, and see what information it gets from where, and you can compare that with your target application. In the end, you will need to learn yourself, instead of asking for step-by-step help here. I presume that "Programming Windows" by Charles Petzold, even if old, is still a good introduction to how windows work. Especially, as I remember that it does not use any fancy object oriented wrapper to the Windows API but creates windows and widgets directly. You have to learn that to go the route backwards and understand the data you get from Win32::GuiTest and what it can be used for.