"hashes in Cuckoo are not thread safe in the sense that they might lose updates when being updated from multiple threads"
Oh, dear. Sounds like a rather significant deficiency compared to Perl, when we are discussing concurrency.
Perl 6 has decided that it is not a good idea to shuffle the inherent problems of updating a data-structure like a hash from multiple threads at the same time, under the carpet. Tieing a hash with a Perl interface to make sure that all updates are done sequentially, is not a good step towards making a fully functional, and well performing threaded program without any bottlenecks. You, as a developer, need to be aware of the issues, and make adaptations to your program and/or the way you think about threaded programming.
Think about writing your solutions as a pipeline, or using react whenever using an event driven model.
In that sense, Perl 5 ithreads makes you a lazy programmer with everything being thread-local by default.
"Note that the Perl 5 solution to updating shared data structures requires tieing and locking."
Wrong. It does not.
If I look at the code of MCE::Shared and MCE::Shared::Scalar, I do see things like a sub TIESCALAR, and &MCE::Shared::Scalar::new being bound to said TIESCALAR. That to me implies tieing. Or am I wrong?
I'm negative about your project because it is still squatting on Perl's name, duh.
I'm glad to hear that it's only the name you object to now.