use Want;

sub fooz : lvalue
{
     defined ($foo_new)?
        $foo_new
        : (want('LVALUE','ASSIGN')?
              $foo_new = $foo
              : $foo);
}

##</code><code>##

use Want;

sub fooz : lvalue
{
     defined ($foo_new)?
        $foo_new
        : want_lvalue?
              $foo_new = $foo
              : $foo);
}