Perl has lotsa possibilities
with variables etc, but imo that makes it somewhat its problem. It's too overwhelming, too many different approaches, too easy to write code no-one with less than 4 years of Perl programming experience is going to understand. Makes it difficult too if you read the code you wrote yesterday and don't have those 4 years of experience yet
That said, I do like Perl, I've even written some irc scripts in Perl (irssi uses Perl as its scripting language) and they actually worked eventually. You can do way more with Perl (threads!), but the learning curve is a lot steeper too than that of mIRC script.
So, here's a feature suggestion: bring out mIRC 7.0 with the possibility to run Perl scripts