What you'd like to have, and I don't know if you can have, is either "several true-threads, all of which therefore operate in one memory-space, or you want a data-structure that exists in a single shared memory-space that is truly attached-to by everyone. I don't know how you do that in Perl.

Basically, what you're trying to achieve here is a read-only shared data-store, and I have no doubt that it is the reference-counting that's causing that multitude of writes. I wish that I understood the whys-and-wherefores of the source code to tell you just how you could achieve “manageable, clean source-code” while avoiding those to-you-useless and unwanted writes.

I frankly don't share the notion that you should use a tied-hash here, because if virtual-memory works for you it sure does have lots of benefits. But not when the VM manager of the operating system thinks it's volatile because you're whacking reference-counters... Those {how's the system to know they're...} unnecessary COW-faults are a serious recipe for thrashing, as no doubt you've seen.