Having investigated further, the test is wrong, and you should just ignore the failures

These are syntactically illegal setlocale() calls on OpenBSD, and so properly return failure. (The standard does not dictate a syntax for these, and different OS's are free to choose whatever they want. The tests use a syntax that is valid on Linux and Darwin.)

On threaded builds, I wrote the code to accept different setlocale() syntaxes, but I didn't want to perturb (hence possibly break) anything on non-threaded builds, which is all that locales had worked under prior to 5.28.

I will fix blead to skip these tests on unthreaded builds on OpenBSD, but I doubt that this problem will be considered severe enough to have the fix make it into a 5.28 maintenance release.

Some further info. I didn't previously fully appreciate that the locale handling mechanism in openbsd is deliberately crippled out of security concerns. So testing of it, thinking it fully works, is going to fail. 5.30 will have new detection facilities for this sort of platform. Alpine linux or anything using the musl libc also has the same behavior.

Thank you for your information and all your effort put into Perl!