I wrote Data::Rmap and do like patches (even suggestions). Using COW in combination with Rmap hadn't occurred to me but I see it could be useful. Enabling it via an option to Rmap would probably be awkward, especially if you could just Data::COW::make_cow_ref the data structure then rmap the COW copy. I'd need to check that it works as I expect but don't have the time today...

If it does work I'll add an example to the docs and leave Data::Rmap doing one simple task.