Hot damn! I figured it out! In studying t/op/magic.t, the failing tests when I build 5.36.3 and 5.38.2, or even 5.30.3, to find a version of Perl that works with PDL-2.084, are focused on various ways to manipulate %ENV. An interesting key is {__NoNeSuCh} which I assume is deep in perlguts and relates to how $0 gets set. The test does `env` but in my infinite wisdom I wrote my own env command and placed it forward of /usr/bin/env, the native env in macOS/Linux. Obviously, now it's clear that mine does not run early enough in process creation, so it fails to capture the true environment needed for those tests to run successfully. When I remove access to my env and let perlbrew use macOS env, those tests pass 100% correctly. Boy, it's really easy to shoot oneself in the foot :-( My apologies to all monks who wasted their valuable time looking at this classic example of pilot error. I will now revisit building PDL.