use Tk;
use Win32::API;

...

# Windows constants
my ($OnTop, $NoTop, $Top) = (-1, -2, 0);
my ($SWP_NOMOVE, $SWP_NOSIZE) = (2, 1);

# 'always on top' state
my $isOnTop = $NoTop;

...

# Create a Win32::API object for SetWindowPos
my $SetWindowPos = new Win32::API("user32", "SetWindowPos", [N,N,N,N,N,N,N], N);
# and one for FindWindow
my $FindWindow = new Win32::API("user32", "FindWindow", [P,P], N);

...

my $w = new MainWindow;

...

my $aotCBut = $w->Checkbutton(-text => "Always on top",
	-onvalue => $OnTop,
	-offvalue => $NoTop,
	-variable => \$isOnTop,
	-command => sub {
		# get a handle to the toplevel window containing our Perl/Tk app
		my $class = "TkTopLevel";
		my $name = $w->title;
		my $NULL = 0;
		my $topHwnd = $FindWindow->Call($class, $name);
		if ($topHwnd != $NULL) {
			# change 'always on top' state
			$SetWindowPos->Call($topHwnd, $isOnTop, 0, 0, 0, 0, $SWP_NOMOVE | $SWP_NOSIZE);
		};
	})
	->pack(-side => "bottom", expand => "yes", -fill => "x");

...

# start the UI
MainLoop;