Reassign value of IN parameter in 9.1.1

This works in 9.1.1 but seems like a bug to me:

create function xout(_x INTEGER)
returns integer
as $$
begin
   _x = _x * 2;
   return _x;
end;
$$ LANGUAGE plpgsql;

select xout(4);

It would not have compiled in version 8.

I came across such a reassignement doing a code review and was surprised it compiled.

Is there a reason for the change in behaviour?