Hi and thanks for all the comments (especially to Cheshire Cat)!

I found that Win32::GuiTest offers the easiest to use methods to find, control and activate windows (FindWindowLike GetWindowText SetForegroundWindow). Easier than the Win32::API-Calls - at least for me :)

To capture the screen I took the SendKeys-Function of Win32::GuiTest. It's not as nifty as Win32::API Calls, but works fine for me. With Win32::Clipboard::GetBitmap() the script pulls the image data out of the Clipboard and saves it to HD. Probably process it with ImageMagick.

Thanks to all again!
Florian