my $activeWindow;

sub Window1_Activate {
    $activeWindow = $Window1;
}

sub Window2_Activate {
    $activeWindow = $Window2;
}

sub Window3_Activate {
    $activeWindow = $Window3;
}

# and so on...

##</code><code>##

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

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