This is a good idea. It's backwards incompatible (in the case where someone really wanted to search @INC for a file named 'HTTP::Request'), but it seems to me within the level of incompatibility that's acceptable for a full version number change. (Though YMMV.)
Re^2: what would you like to see in perl5.12? (require $class)
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Do you mean %INC? Changing %INC to match is not required, and I wasn't suggesting that it should be done. People who want to search %INC (or @INC) can continue to do the same class to path conversion they are already doing.
But if you wanted to change %INC to class names, you could maintain compatibility with old scripts by placing both the class name and paths in %INC.