partial assignment using lvalue?

dear all,

it seems all the "lvalue sub" RFCs haven't mentioned this.
currently we can do

  $str = "foo bar";
  substr($str, 4, 0) = "baz ";

and $str will become "foo baz bar". should we be able to do 
this with lvalue-subs? how can one returns "pointer" to a 
portion of a scalar?

kenneth