use Tk;
$mw = tkinit;

$mw->optionAdd("*tearOff", 0);
$m = $mw->Menu
    (-menuitems =>
     [
      [Cascade => "~File", -menuitems =>
       [
    [Button => "~Open"],
    [Button => "~Reload"],
    [Button => "~Quit"],
    [Cascade => "~Cascade", -menuitems =>
     [
      [Button => "~Foobar"],
     ]
    ]
       ]
      ]
     ]
    );
$mw->configure(-menu => $m);

# Disable Foobar:
$m->entrycget(0, -menu)->entrycget(3, -menu)->entryconfigure(0, -state => "disabled");

MainLoop;