Ok, getting back to the line I posted (which is in DateTime::TimeZone::Floating, not my own code), there's nothing in the documentation which suggests that this failure in this line is deliberate. After all it is just a "use parent" statement and has no wildcards or patterns in it.
Given that there is nothing dynamic in the strings involved, given that they work everywhere else (including two other Perl 5.19.9 systems run by the same tester on different versions of FreeBSD), this either has to be a bug in Perl (or a dependency linked to somewhere) or it has to be a broken parent pragma (which iirc ships with core).
My reason for contacting the tester in this case was to hand off what tracing I had managed to do so that if he was interested in taking it further he wouldn't have to duplicate my work. That a deliberate change occurs in related areas of code (tainting) in no way means that new bugs aren't introduced in that process.