I doubt the ability to override config using env vars is gone

Oh yes - that's still there. I just don't know how easy it is to get things set correctly for the switch from 'gcc' to 'cl'. (The switch in the reverse direction used to be trivial - happened automatically.)

IMO, the jury is out until someone comes along and actually does what the OP wants to do, by overriding the ActivePerl-5.20 defaults.
*Then* we get to find out how trivial (or otherwise) it is.

Cheers,
Rob