Perl warnings and the warn function

“Warning!” by John Goodridge is licensed under CC BY-NC-SA 2.0

I men­tioned in pass­ing last week that the next major release of Perl, v5.36, is set to enable warnings by default for code that opts in to use v5.35; or above. Commemorating Perl’s 34th birth­day the week before that, I not­ed that the warn­ings sys­tem has been get­ting ever finer-grained since its intro­duc­tion in 2000. And fel­low Perl blog­ger and CPAN author Tom Wyant has been cat­a­loging his favorites over the past sev­er­al months — the lat­est as of this writ­ing was on the ​“ambigu­ous” cat­e­go­ry of warn­ings, and you can find links to pre­vi­ous entries in his series at the bot­tom of that post.

It occurred to me after­ward that there may be some con­fu­sion between the warnings prag­ma and the relat­ed warn func­tion for report­ing arbi­trary run­time errors. warn out­puts its argu­ments to the stan­dard error (STDERR) stream, or if it’s not giv­en any then you get a string with any excep­tion from $@ ($EVAL_ERROR under use English) fol­lowed by a tab and then “...caught at <file> line x.” If that’s emp­ty too, a plain warn just says, ​“Warning: something's wrong at <file> line x.”, which isn’t exact­ly help­ful, but then again you didn’t give it much to go on.

warn out­put doesn’t have to go to STDERR, and this is where the rela­tion to the warn­ings prag­ma comes in because both are gov­erned by the __WARN__ sig­nal han­dler in the %SIG…