I think that's exactly what I'm seeking to do in Using functional programming to reduce the pain of parallel-execution programming (with threads, forks, or name your poison)

Here "hashmap" would appear to be the parallelisable function, which is (I hope) side effect free. I have a similar function I'm playing with (not mentioned in the post) which I'm calling vector_reduce. This can't be parallelised.

The two functions together seem to be the conceptual core, or something close by, of MapReduce, made famous by google, and newly on cpan.