The converse to split is the join function, which takes a list of strings and joins them together again.

No. The converse to a split is a regular expression (with matching). The idea here is that with matching you specify what you want to match and you get that as a return value ala @simpsons = "Bart Lisa Maggie Marge Homer" =~ /(\w+)/g). With split you specify all the things you don't want and you get everything else. Which one you use depends on what is more natural to specify.

I don't understand what meaning of converse you use here. English is not my first language - but here is what I found for 'converse' in the Meriam-Webster online dictionary:

something reversed in order, relation, or action: as a : a theorem formed by interchanging the hypothesis and conclusion of a given theorem b : a proposition obtained by interchange of the subject and predicate of a given proposition <"no P is S " is the converse of "no S is P ">

I believe the OP used 'converse' as a synonim for 'inverse' in the mathematical sense, i.e. he wanted to say that  split o join = identity. Which is quite close to being correct.

Or perhaps you meant join in the first sentence? This possibillity occured to me just after posting this comment.