Welcome to the forum ^
Linking does not offer protection if you don't make the connection.
If your first flight is late CX has no obligation to get you to Shanghai on later flight. You are taking the risk
If made as ONE reservation/PNR and your first flight is late CX has an obligation to get you to Shanghai on later flight. CX are taking the risk.
50 minutes is not long if you have to go through immigration/collect bags then check in & go through immigration again. Worth asking CX Perth by phone to ask if they will tag bags for both flights in Perth.
Personally if CX would not tag bags I would be booking as 1 PNR. 1hr 50min would be different. Is there a later Shanghai flight you can book ?