I mean... mIRC isn't a bad client (it's still my favorite) but why does it lack such a important feature?
It's not important. Use Klient if you want to script in Perl. You are right that not everyone likes mIRC sripting but not everyone likes coding in Perl either. It is also un-necessary and perhaps over-engineered. mIRC's scripting language is quite powerful enough for the job required of it.
