You use the id method to get X11 window ID or Win32 HWND for a widget:
$id = $widget->id; print "$id\n"; # Prints 0x9c944c
This returns a hex value. A common use for this method is to embed a Tk window in another application. The PerlPlus Netscape plug-in does just this. It is described in Chapter 22, "Perl/Tk and the Web".
$path = $widget->pathname($id);