Comments on anything and everything I've posted so far.

Random comments:

• I don't think separating out the module builder and installer will be a big issue. There's gonna be a whole lot of common code between the two so you'll probably end up with them in the same distribution anyway..
• I wouldn't worry about C6PAN/CPANPLUS. These should be independent of the module builder/installler. People are always going to want to build/install modules outside of the context of CPAN. The only issue I would have is to make sure that there is a way to get at your META.yml equivalent without having to execute code.
• Might be worth looking at Ruby Gems (one of Ruby's solution to this problem). The chapter on Gems from the Pickaxe book is available for download if you want a overview. There are some interesting design decisions.
• Dig out the Module::Build mailing list archives :-)