You can do it with a z variable that you can think of as the distance into the screen. Then you just change the object's size based on how far into the screen it is to make it seem farther away. Something like set scale to 100/z would work but you can tweak the number. If you want farther away objects to be behind closer ones you could do that with a for each ordered condition.
Here's a capx I did a while ago that utilizes that effect.