And the only way I can change it, other than editing each instance of Makefile.PL, is to recompile Perl? Is that right?
No - if it's set in Config.pm, amending that setting to whatever you want should be all that's needed.
In Windows, it's specified only in lib/Config_heavy.pl. I don't know which file(s) specify it in MacOS but I expect it would be lib/Config.pm and/or lib/Config_heavy.pl (if the latter even exists on MacOS).
For a quick check to see what it's set to you can just run perl -V:Author