JAL flight Perth/ Tokyo

missing the narita to sapporo leg of a return flight so I can stay in tokyo.Okay?

No.

If you miss the first part of an itinerary, the rest is automatically cancelled. You need to call your airline and change the reservation, or change your plans.

Exception is if your Sapporo-Tokyo flight was made separately from the Tokyo-Perth flight. Then it doesn't matter if you miss it.

Sorry, that should be Sapporo-Perth not Tokyo-Perth. But the rest stands.

Basically if you no-show your flight, the remaining flights on your itinerary are automatically cancelled. Ticket rules vary, but for most discount tickets there are no refunds in such cases and no rebooking, or if there is, there is often a substantial penalty.
In any case, you should know your ticket rules on refunds and date changes before you commit to buying any ticket.