use Win32::GUI();
use Win32::Capture;

my @WIN = FindWindowLike('hier kommt der fenstername rein'); # Find the HWND to be captured.

if ($#WIN < 0) {
    print "Not found";
} else {
    foreach(@WIN) {
        my $image = CaptureWindowRect($_, 2, 0, 0, 400, 300);
        $image->SaveToFile("screenshot.png");
    }
}