I like your approach too though.

So we have a s/// (substitution) method, and a join / split / map method. As you mentioned, tr/// isn't a good candidate, but I couldn't resist at least tossing out there the thought of tr/// applied to the first letter of words by using pos along with m/\b[a-z]/g (to locate position of first letters of words) along with substr (as an lvalue to constrain the effects of tr///). ...it could work, but sounds like madness. ;)

TI(always)MTOWTDI