I don't think you'll find anything inside mIRC was created to infect users with viruses. I guess Internet Explorer, and all other browsers, should disable their ability to download files. ActiveX and JavaScript should also be banished from the Internet, aswell as...well, actually, let's just ban the Internet!
The Internet, and IRC along with it, is not a safe place for computers. The real world isn't safe because people like to break the law, and the cyberworld is no different - people are assholes, and they think being assholes is the best thing since sliced bread. This is said over and over again, both generally and with details. If people cannot be bothered to read then God knows why they're using IRC. Look at the amount of people who think mIRC/IRC are the same thing - if they even bothered to read the big yellow box on the front page of this website, they would see mIRC/IRC are two different things. They don't even read that!
I've seen a number of scripts use $decode, sometimes for script-to-script talkers, or just for general fun. Even had a serious use for it once ^_^
Kelder: It is unfeasible - but, oh to dream!
Regards,