You said...
Well, I don't quite follow your explanation (I'm not sure where set_message() appears - it's not in your sample code), so instead I'll just describe what happens.

Ooops... I forgot that I provided a code snippet rather than my "real" code... I started with the template that's in the CGI::Carp doc, which illustrates how to use set_message() by setting a function. In my live code, I use an if statement to test if the browser has a cookie that says whether to output error text, but forgot to include it in my code segment (to keep it brief). That's what I was referring to in my reply that you couldn't reconcile. Anyway, your explanation was perfectly clear, and confirms my explanation as well.

I will include my entire BEGIN block in a separate posting (which you can find in posting #421127), which takes a slightly different tack than my question here. The reason is that I have a separate, unrelated question.