Some of this seems readable to me (someone who's never seen APL before), but some of this is just wild:
→0⍴⍨0∊⍴i←⍸127<c←⎕UCS w
But to me as a relatively experienced APLer, it reads very clearly as "return if there are no non-ASCII chars in `w`" (and if there are, the indices of those chars will be stored in `i` and the code point of every char will be stored in `c`).