The pertinant man page is actually Tk::Wm which tells you how to communicate with the window manager.

Example: (Functional for both XWindows and MS-Windows)

use Tk ;

my($mw) = new MainWindow ;

my($closeSub) = sub {
    print "closing\n" ; 
    $mw->destroy  ;
} ;

$mw->protocol('WM_DELETE_WINDOW', $closeSub) ;

my($button) = $mw->Button(-command => $closeSub, -text => "Close")->pa
+ck ;

MainLoop ;
[download]

This should also work for 'Toplevel' window created with $mw->Toplevel ; $topLevel->protocol(...) ;