Indeed, sorry for not reading more carefully. However my comment isn't entirely void, have a look at Apocalypse 6 where Larry says that lvalue subs will not get to see their rvalue unless they use tieing

If you need to do pre- or post-processing on the "public" value, however, you'll need to return a tied proxy variable.

and he also said that the tieing will be easy. He has other plans (temporizing - basically a tie interface to local) that would break if it was done the way you want.

There's still also the issue of potentially surprising order of evaluation.