The attitude that "PERL" is absolutely incorrect flies in the face of this somewhat, wouldn't you say?
If one wants to be absolutely correct, one should follow the usage in the OED, which does not say that "PERL" is incorrect, but that it is irregular.
Of course, any true linguist (I'm probably not speaking about your high school English teacher here) will tell you that "correct" is whatever communicates what you want to communicate. The fact is, everyone here agrees that "PERL" sometimes communicates something you don't want it to communicate. We just can't agree on whether that's a bug or a feature. :-)
And that's essentially the meaning of "irregular". A quite useful category, if you're writing a dictionary.