You can connect with QF77 from PER which does get into SIN in time to connect with the JL SIN-KUL flight.

that would be :
SYD-PER - QF 565
PER-SIN - QF77
SIN-KUL - JL721

Earning miles OW all the way

I've done the MEL-PER-SIN-KUL trip a number of times recently and it works pretty well. If I dont need to be there the same day, I would just take the direct flight SYD-SIN - overnight at the Crowne Plaza Changi Airport, then catch a morning flight up the following day (usually on SQ, MH or MI rather than wait until the JL flight that evening)