It's not a rule I can remember encountering anybody actually following, but it's still theoretically a thing at least up to about a decade ago, which is very definitely perl5 era.
It's not where I thought it was in the perldoc though, so either it isn't in there at all or I screwed up looking for it.
So, roughly, "I think it turns out we're both sort-of wrong" :)
So nobody pays attention to that rule anyway.
I do get slightly confused switching between #perl and ##prolog on freenode sometimes, but the other regulars just laugh at me and we move on ;)