Is this simply a case of "bad expectation" on my part ?

Hmm, it may depend on how long ago FLT128_MAX was invented/added to POSIX standard .... POSIX module is known to be behind the current standard ("recent" socket6 stuff or some such)

https://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=FLT128_MAX

https://encrypted.google.com/search?hl=en&q=site%3Ahttp%3A%2F%2Fmetacpan.org%2F%20FLT128_MAX