I havn't delved much into the inner workings of menus, but I have one observation, which may be a clue. When I run your program, I click on FILE and the new submenu is automatically opened up( which I guess is the problem you describe).

It looked to me like if there is a submenu, as the first element in the dropdown, it gets automatically selected. If I move the New submenu code, out of the first position, it works fine.

I would guess that you could find a workaround if you need the $new first in the list, maybe force the default selection to be other than the first. Or just take the easy way out, and put your submenus towards the bottom. More help may be available on comp.lang.perl.tk