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.
