my $menubar = $mw->Menu(-type=>'menubar');
$mw->configure(-menu=>$menubar);

my $menu_datei = $menubar->Menu(-menuitems=>
                                [
                                 ['command' => 'Menupunkt1',
                                 -command => \&callback,
                                 ]
                                 ]
                                 );
$menubar->entryconfigure('Datei',-menu=>$menu_datei);

my $neues_menu_datei = $menubar->Menu(-menuitems=>
                                        [
                                         ['command' => 'Anderer Menupunt',
                                          -command => \&callback2,
                                         ]
                                        ]
                                      );
                                      
$menubar->entryconfigure('Datei',-menu=>$neues_menu_datei);