Much as I like the intent, package main; looks ugly as sin to my fair eyes! We must be able to do better than that, surely?

To me, something like require v5; should trigger the "uh, this isn't Perl6" switch - and is much more obvious to my mind. "I require version 5" is better than "I live in the main namespace" (or whatever package does :). I'm aware that require doesn't put a ceiling on the version, but I doubt adding one now will really make any practical difference.