$menu->entrycget(0, -menu)->entryconfigure(1, -state => "disabled");

#!/Perl/bin/perl

use strict;
use Tk;
use Tk::Menu;

my $mw = Tk::MainWindow->new();

my $var = 1;
my $menuitems = [
    [Cascade => "~Datei", -menuitems =>
        [
            [Button => "~Neu", -command => sub{ return 1; }],
            [Separator => ""],
            [Button => "~Öffnen", -command => sub{ return 1; }],
            [Button => "~Sichern", -command => sub{ return 1; }],
            [Checkbutton => "~checkbox", -command => sub{ return 1; }, -variable => \$var,],
        ],
    ],
];
my $menu = $mw->Menu(-menuitems => $menuitems);
$mw->configure(-menu => $menu);

$mw->Button(
    -text=>'Knopf',
    -command => sub{
        if( $var == 1 ) {
            $var = 0;
        }else{
            $var = 1;
        }
    },
)->pack();
$mw->Label(-textvariable => \$var)->pack();

MainLoop;