Even if the post is quite old, here it is how to modify the Application menu, for example the default "About my application", using Tcl::pTk and Tcl::Tk

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

    my $int = $mw->interp;
    my $menuPATH = $int->widget($menubar);#getting Tk name of menubar
    $int->Eval("
        $menuPATH add cascade -menu [menu $menuPATH.apple]
        $menuPATH.apple add command -command {tk_messageBox -message \
+"Copyright XXX\"} -label \"About My Application\" 
        $menuPATH.apple add separator
    ");
    #rest of the menu comes here

       $toplevel->configure(-menu => $menubar);
[download]