Filtering a hash to produce a subset-hash not documented

I'm struggling trying to filter a hash of hashes -- to reduce it to a hash where the value-hashes all contain a particular key. The example can be seen in [my question on Stackoverflow](https://stackoverflow.com/questions/46306363/). A similar (?) [question](https://stackoverflow.com/questions/41579581/) is answered with a depressing answer to the effect, this is not possible -- until #105 is merged.
I hope, I'm misreading it and what I'm seeking is already possible -- just lacking an example...