If you want single pass, then you have to do it on a per block basis at the most (e.g. C89).

But this is not meant to be a fully fledged language, it's meant to be a teaching tool. If you want a fully fledged language that allows for out of order declarations, try Odin!

Also, the syntax of Oberon/Pascal doesn't really allow for it in a nice way. It kind of looks really weird when you allow for it out of order.