Dear Daisuke, I think your new module is a great addition to Params::Validate but I think the naming should imply it's relation to Params::Validate.
Perhaps it should be named Params::Validate::Derivable or something along those lines. If nothing else, the one line description in the NAME section should change from: Provide Subclass-able Validation Mechanism to Provide Subclass-able Validation Mechanism for Params::Validate Regards, -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.