First, I don't have a c:\perl

You are right.
The post to which you have replied was in response to GrandFather's issue - which was a totally separate issue to the one that you (and I) experienced.

I wonder what cpanm does that plain old cpan didn't??

Interesting that cpanm gets the job done. It works for me, too.
I don't know how it avoids the problem that the cpan utility encounters, and I couldn't spot any bug reports that seemed relevant.
I was thinking that it must be a bug in the cpan utility, but that is NOT so - because when I try to build PAR-Packer-1.050 the ol' fashioned manual way (without using the cpan utiity) I get the same 6 test failures.
It's a bug in PAR::Packer's t/20-pp.t script, and the use of the cpanm utility is currently the only way of working around that bug.

This bug in PAR-Packer should be reported.
We know that it's limited to Windows 10. At least we know it doesn't afflict Windows 7.
We probably ought to determine the earliest version of PAR::Packer that exhibits the bug.
I don't have time for this right now ... perhaps I will later on ... or perhaps someone else can follow this through.

Cheers,
Rob