Still probably not what you are looking for (i.e. no refs)

The refs were only a half-arsed attempt to get thing to work when I suspected that the @_ aliasing was responsible for "undo" my changes--a total wrong guess!

And, your post has moved me along considerably. Your insight of using the same substr as an lvalue into which to assign the return from the recusion--with the same parameters as are used to supply the substring to that level of recursion is (I think) the solution.

I've put that into the real code and it gets me closer to the desired result. At this stage, the rest of the problem appears to down to my selecting the correct substring--ie. a completely different part of the problem.

So thankyou. Very much.