... compilation messages and metadata statistics for use by frameworks (e.g. Catalyst and Moose).

Actually, Moose does not and never will use attributes. As perrin says below, they are best avoided at all costs. The API to use them is horrid and the limitations of that API are (IMO at least) totally unacceptable. Even Catalyst will eventually do away with attributes in favor of using something similar to how Moose works or something more radical like Devel::Declare.