Use ppm. From a command line, run ppm install --nodeps Win32::Screenshot and it'll install and work just fine. The dependency checker can't find perl-magick, but that won't be a problem when you go to use it. Just make sure you've got these use directives:

use win32::API;
use Image::Magick;
use Win32::Screenshot;
[download]

and yes, they're case sensitive. I couldn't get the CaptureWindow() functions to work though. They capture the entire screen, not the window, but that shouldn't be a problem with what you're doing. Good Luck!