At the moment, MoarVM has a potential for crashing ... So, to work-around the possibility of crashes, one should make sure that the hash already has all of the possible keys before starting to do the parallel run. If rewritten your code to be more idiomatic Perl 6
This is rather amusing if sadly unsurprising. Having been sold Perl6 on the basis that is has all the benefits of using a bytecode VM, we now see that the VM tail is wagging the Perl6 dog.
Surely the sane advice should be: if a particular VM is unstable then stop using it and use another VM instead. There's no need to rewrite/refactor already working code just because one VM has bugs.