Ok, I'm convinced; it's a bug. The fix does not look hard, but testing that it has not broken anything else is less so. I've never managed to do a cpan install of Tk without using "force" because of the number of errors the build throws up--even though it seems to run fine--so determining if anything new is broke would be painful.

I've taken remiah's basic approach so the problem is now "avoided" and I get the '[]' chars in my menu items. Life is good.