Thanks for letting me know about /var/log/setup.log & /var/log/setup.log.full. I spent a few minutes looking through them and found this:

2021/04/19 19:30:44   19 install perl_base                            
+ 5.32.1-1               
2021/04/19 19:30:44   20   erase perl_base                            
+ 5.30.3-1
[download]

and this:

2021/04/19 19:30:44  150 install perl                                 
+ 5.32.1-1               
2021/04/19 19:30:44  151   erase perl                                 
+ 5.30.3-1
[download]

and several other similar pairs of messages. So it appears cygwin deleted 5.30.3-1 when it installed 5.32.1-1, I just didn't read the cygwin setup page closely enough before clicking "Next". It appears all of the modules I installed while using 5.30 are still located at /usr/local/share/perl5/site_perl/5.30/, it's just that some core components of perl 5.30 appear to have been deleted.

I will use Perlbrew to manage multiple Perl versions from now on.