Why doesn't X-Chat Aqua support MacOS X 10.2 or less?

This is because the perl plugin no longer works with the version of perl that comes with 10.2. Although X-Chat Aqua doesn't strictly rely on the perl plugin, I didn't like the idea of releasing an xchat variant without it. The final straw in this decision was that I have started to take advantage of new features in 10.3+ thus making supporting 10.2 impossible (or more difficult than it's worth).