G'day Rob,

"Are you able to verify that the abstract does, in fact, =~ m![[:cntrl:]]+! in perl-5.20 but not in perl-5.26 and later ?"

The short answer is "Yes".

The longer answer is tad more complicated because the code that would demonstrate this is from $work. I basically ran a GitLab CI/CD pipeline with 14 jobs. Each job used a different docker Perl image: perl:5.16 to perl:5.36 plus three others to cover deltas that indicated some backward-incompatibility in subversions (follow the footnote link in the OP for more details). The output from those jobs only showed the warning in v5.20 to v5.24.

There was only the one source Perl module. It was very old, created long before I joined the company, and every line ended with a CR-LF. When I removed the CR, the warnings disappeared for all the v5.20 to v5.24 jobs; the output for the remaining jobs was unchanged.

I suspect the module was originally written on an MSWin platform for ActivePerl/Strawberry Perl/whatever and then migrated to a Linux box without any line-ending conversions (but that's very much a guess).

And, as a further guess, the way $self->{ABSTRACT} is assigned its value, has something to do with it.