No idea. Without a reproductible test case I can't even try to make a guess.And I don't know XML::LibXSLT module code, can you try to first get the authors of the Perl modules to look at it,
James -I have no such problems with registered Perl functions using the current libxml2 or libxslt with XML::LibXML, but using the developer release (http://search.cpan.org/~phish/XML-LibXML-1.58_1/). I'd be sure you're using it.