I don't think this would not be a significant cost. Only the package declaration would have to be checked ...

Is that true? What about?:

::::::::fred = 1;
[download]

But also, once you start trying to validate this, what other additional rules are you going to add? Should we allow package _::_::_;?

That said, I'm fine with Perl's "if you want to do something stupid, who are we to stop you?" philosophy.

I also think that we should stick to that principle.