As far as the whole "ask the script author" comment. I understand we are here to help with mIRC and not scripts other's have wrote, but a quick look at the result, and the behavior described by the OP indicates not a problem with the linked 'tool' script, but rather something unexpected within mIRC happening.
The OP did not include how they were using the script, so without prior knowledge that mIRC throws an error like this with /msg (and not with /echo or /write) there is no indication where the problem lies. That's why I asked what he was doing, and allowed for the possibility the problem was from either source.
It also turns out actual commands are not run anyway and the rest of the line is not evaluated, so there is no vulnerability.
