all will be well if every program does use Foo; that is never, ever preceded by use Foo::Bar;

I disagree with the two underlined wordings. It's possible that the two definitions of package Foo::Bar don't conflict, and that the overlap may even be intentional, for example if package Foo needs to monkey-patch something into Foo::Bar. Granted, these cases may be rare, and the point that the packages might conflict is very valid, but the above wordings are just too absolute for my taste.