Silly me. Of course something is on my @INC, or I couldn't run perl. The first JSON:PP on my @INC is v2.24. But this doesn't explain why CPAN keeps seeing that instead of its own modules after doing an install. Does CPAN just assume that its own module directory is at the head of my @INC ? It isn't even there at all.

Which is even more mysterious, because if CPAN is finding JSON::PP on my @INC, it shouldn't find new modules at all after I install them, as they are in my CPAN directory and not on my @INC.

(To access modules that I install with CPAN, I have to change my PERL5LIB immediately before running perl, and change it back immediately afterwards, because of incompatibilities with our wrapper for git.)