if you mean which Win32::GUI window is the active one, you can simply track this with the Activate() events. an example:

my $activeWindow;

sub Window1_Activate {
    $activeWindow = $Window1;
}

sub Window2_Activate {
    $activeWindow = $Window2;
}

sub Window3_Activate {
    $activeWindow = $Window3;
}

# and so on...
[download]

if you instead mean any window on your screen (and you don't need realtime monitoring -- which will hog your CPU), then I would do it setting up a timer object:

my $checkActive = $Window1->AddTimer(
    -name => "checkActive", 10 
);
# note: 10 milliseconds means 100 times/second

sub checkActive_Timer {
    $activeWindow = Win32::GUI::GetForegroundWindow();
}
[download]

hope this helps...

cheers,
Aldo

King of Laziness, Wizard of Impatience, Lord of Hubris