RE: 5.6.0 BUG: Lexical warnings aren't lexical

RE: 5.6.0 BUG: Lexical warnings aren't lexical
Message-ID: <000701c0b56e$73944220$07bdfea9@bfs.phone.com>

A variable will be checked for the "use once" warnings if:

1. It is in the scope of a use warnings 'once'
2. It isn't in the scope of the warnings pragma at all AND $^W is set.

Otherwise it won't be checked at all.

Part 1 is what is in perl >= 5.6.0, Part 2 is what I'm fixing.

The enclosed patch partially fixes this issue. What I didn't
get to work was the case where the "used once" warning is enabled
in any file other than the main file.

p4raw-id: //depot/perl@9401