If you are making a through booking of Perth to Dalas/Forth Worth - no issue
it looks like you are making 2 separate bookings though, I wouldn't risk it unless the SYD-DFW ticket is a flexible fare with no change penalties / fare differences and a good chance of availability on a later flight - 1 hour delay and you could be looking at quite an expense
How much would it cost to change the SYD-DFW flight to be PER-DFW via SYD ?