Thanks for your reply. It is helpful.
Looking at the line (in a dependency) that is throwing the error and the documentation, I am entirely hard-pressed to see this as deliberate. The line which is throwing the error is, again,
use parent 'Class::Singleton', 'DateTime::TimeZone::OffsetOnly';
There is nothing dynamic in either string. There is no pattern matching going on. And the same tests are passed on two other 5.19.9 systems. If this is a deliberate change, it is not conforming to any reasonable interpretation of the documentation I can think of.