I've been using XML::SAX::Writer to spit out some XML, and I couldn't work out why the prefix wasn't getting written on the start element. In the end I extracted out a small bit code and made a test case that continued to fail. The prefix and namespace are quite long and hence were defined as constants at the top of the script. Just before I was about to mail the perl-xml list shouting BUG! BUG! I decided I'd better shorten this prefix and uri to something a couple of chars long so I can format the xml the writer spits out in 78 chars easily.
And then it starts working.
Like a bolt of lightning I suddenly realise that the prefix was starting with 'xml', which of course is reserved in the spec. You can't start tag names with xml. Of course, I hadn't noticed it as I'd been spending the whole time staring at the code that was creating the tag which had constants like PREFIX and NAMESPACE rather than the actual values.