I would never have found that without your code

Then you need to bookmark these and use them :)

Basic debugging checklist , brian's Guide to Solving Any Perl Problem, CGI Help Guide , Troubleshooting Perl CGI scripts ... More generic advice :) On debugging, verify everything, talk to teddybear ... checklists and more

"Why did upgrading my CPAN modules trigger this change? Why didn't this show up before?"

Because of the source of the code you're not sowing ... somehow poked/triggered/exposed/changed ... something to do with one of use/require/eval/do/base/AutoLoader/source filters/coderefs work in @INC, $PATH/%ENV/PERL5OPT...