Good points in both above posts. As to Clone::PP, glancing at the docs it says it'll call clone_self to do cloning for the object in question and/or clone_init after the clone's done. Of course you'd also want to look into STORABLE_thaw for Storable and Thaw for FreezeThaw for completeness.

As for forests, Ruby also provides something similar where you can do ObjectSpace.each( MyClass ) { |instance| ... }.