The tourists resume at 4-180 in reply to Australia's 385, with Ben Stokes on 14 and Ian Bell nine.
The first two days have been played in extreme heat, with the temperature reaching 40.4 degrees on Saturday.
The mercury is tipped to reach 41 on Sunday and Australian bowling coach Craig McDermott predicts the baking sun will have a significant effect on the pitch.
"I think the cracks are going to get bigger, there's no doubt about that," McDermott said.
"They grew quite a lot (on Saturday). And that will continue.
"There's a couple in line with the stumps, so that's going to play havoc on batsmen's minds.
"It may never even hit those cracks. But again, that's a psychological thing from a bowler's point of view.
"We've just got to play the best we can on that wicket. But it may be better that we're bowling last on it because the cracks will be even wider, so it will be great."
England danger man Kevin Pietersen is already back in the shed after he fell to Peter Siddle for a record 10th time.
Australia can reclaim the Ashes with a win in Perth, but McDermott scoffed at talk that Pietersen's dismissal signalled the end to England's challenge.
"Bell's still in and has an unbelievable record," McDermott said.
"Prior nearly averages 50, or just on 50.
"So they've got some good batting to come.
"We've got to make sure that we start the day as we finished yesterday - on the ball."
Australia can take the new ball after 12 overs on Sunday and McDermott said it was important for his bowling group to use it wisely.
England opener Michael Carberry said his team needed to get as close as possible to Australia's total.
"Getting near 380 for us is the long-term goal," Carberry said.
"What we're trying to do now is break our run chase down into small targets - getting through that first hour, getting up to 200, and taking it in small targets until we get to 300, 380."