I said the result was read-only. I never said $r was read-only. Oops, I didn't original post, but I did in my second post. Fixed.

The OP wanted a means of accessing the value at "key" @k.
Using the "$r" method, one cannot edit the accessed value.
Using the "$p" method, one can edit the accessed value.

When something cannot be edited, it is said to be read-only. So while the scalar $r itself is not read-only, the accessed value is read-only.

Update: Added underlined text.