After several hours of puzzling through the XS code for XML::LibXML (and WOW is my XS-fu rusty!), I gave up and filed an RT ticket on it. I gave it all the info I had, including the specific test-suite that was generating the error. One thing I didn’t provide was the location within the test suite, because that seemed to change from platform to platform.
Well, great kudos to Shlomi Fish, who has already found and quashed the bug! It was definitely more complex, and buried more deeply, than I thought. It was related to my use of the XML::LibXML::CallBack class, usage of which was new in this release (there’s an issue with the slightly out-of-date libxml2 package that MacOS X Snow Leopard has, that interfered with a simpler solution, which forced me to use the callback class). Anyway, thanks to everyone who responded! You got me deep-enough into the code that I was able to craft a better bug-report than I might have otherwise. And hopefully, Shlomi is as happy to have the bug be found and dealt with as I am!