Oh I don't know about that. Actually, I think you *are* thinking about it properly. Although it makes sense that external code will refer to certain variables, it is natural to expect that said code won't walk all over those variables without restoring them or at least telling you about it in some way. File that report, in my opinion.
Agree with this one. Nobody shall assume that $@ or $! or $? would keep their values for ever. It would be tedious job trying to properly localize all of the variables that your code may indirectly affect.