Let's break this down into steps. First, you need to get the window IDs of the buttons that you want to click on. You've got two routes. Find them yourself using utilities like I had suggested or using Win32::GuiTest to write your own code to dig through the structure of the program. Or you can get the information from the program's source code (or from someone maintaining that source code). I really don't see how you can get around that.
Next, there's the clicking of the buttons. Not trying to be mean here, but have you taken the time to read through Win32::GuiTest's documentation? You keep mentioning only PushButton and MouseClick functions. If you take another look at that documentation, you'll notice that one of the first functions it describes is one called SendMouse.
Again, I'll point you to take a look at the code I posted in Re: Problems with WIN32::GUITEST. Look at the section for the 'mouse' emulation. That section is using SendMouse to click on the buttons. If you run that code, you should see the mouse cursor moving to the buttons. Try using something like that with your code.
If you still can't get the button clicks going, try writing some short code to push buttons on the calc.exe from Windows and you can post that code to get more help on grasping the basics concepts.