Perception is more than sight, though. If I can hear you behind me, I can cast a spell at you. Deductive reasoning isn't perception, though, if you can't be heard or otherwise noticed, and you are behind a magus attempting to target you, you haven't been perceived. Make sense?
If the Dimicatio field is bounded (and I've always thought it was), and you're at the edge, he can't go behind you without going out of bounds, which probably results in a DQ. If you're not far from the edge he may be close enough to still perceive, by other means, feeling the air swirling behind you, hearing his huffing breath from having quickly cast a spell. A spell like PoF and BoAF where the effect of traveling to the target is technically cosmetic may still target the person as long as they are sensed. Does Wizard's Leap have a popping sound associated with it? Probably. Does it require some sort of awareness check to have the PoF or BoAF reacquire the target, probably? I'm thinking Wizard's Leap may not be an effective defense in Dimicatio, unless it's on Rego spells not after the fashion of Vilano. I don't think Vilano spells are legal in the Dimicatio, because they inflict damage and the spell effect ends before the stone ever gets to the target. And finally, IMO, Dimicatio is about going first. If you can't go first, you're probably toast. Rarely do they volley back and forth. You don't want it to, as it means others are going to figure out what your tactics are further on down the line.
The Melee in Normandy Tribunal is not the same as Wizard's Melee in Houses of Hermes. The Melee is a mundane event similar to capture the flag. CrIm castles are created with Ring duration at opposing ends of the field the goal is for the grogs to disrupt their opponent's castle first. It is non magical, except for the castles created.