the following Cwd version acts in the opposite way

Heh ... that's pretty flakey, too. (I see the same.)

What I'd like to find is some code where the behaviour changes between perl-5.32 and perl-5.34.
Apart from t/01-basic.t, everything I've tested (including your fakeOS.pl) is doing the same thing on both of those versions of perl.

If I take your fakeOS.pl, change $^O = 'Unix'; to $^O = 'MSWin32' and run it on Ubuntu-20.04, I get:

$ perl fakeOS.pl
Use of uninitialized value $abs_path in concatenation (.) or string at
+ fakeOS.pl line 10.
[download]

and

$ perl -MTest::Harness fakeOS.pl
/home/sisyphus/pscrpt
[download]

Again, however, it's the same behaviour on perl-5.32 and perl-5.34.
Interestingly, that error message when run without loading Test::Harness hints at being perhaps connected to the failure that t/01-basic.t is triggering on perl-5.34 and later.

Dunno ... I think I just need to find the time to make one or two bug reports and let the experts sort it out.

Cheers,
Rob