Although I agree that join and reduce have similar parameter signatures, the semantics of the two functions is very different.

join concatenates elements together with a separation string sandwiched between all elements. That's it.

reduce does arbtrarily complex mangling of elements to produce a single result. It could be anything.

Technically, a join is a special type of reduce with a string and an implied coderef. But join semantically implies not mangling the elements, but simply concatenating them with a bit string. I think that association is so ingrained in the minds of perl programmers that you need this specially named reduction as a separate function called 'join'. Save reduce for the general, functional version.