I didn't say prior to setting it off. I said to avoid setting it off.
P 417 CRB "Creatures that succeed on a Perception check detect a trap before it is triggered."
If you sense it before it goes off, you wouldn't set it off unintentionally. It's not just to get a reflex save or something. You get a reflex save against things you aren't even aware of... like an invisible caster throwing fireball at you.
"Most Perception checks are reactive, made in response to observable stimulus." "Perception is also used to notice fine details in the environment. The DC to notice such details varies depending
upon distance, the environment, and how noticeable the detail is." So factoring for distance, conditions, distractions can increase the DC. Running down a hall being chased by an enemy will distract you and make it more likely to set off a trap...
To me, the comparison would be that you get a perception check before combat even if the opponent is invisible. If you fail, you don't act in the surprise round. If you succeed in seeing/hearing/smelling him... combat will proceed by initiative.
So, even against hidden things, a reactive perception check is the norm. Only 6 of the core classes have perception as a class skill, so a lot of PCs will be in trouble still.
As far as our discussion about the trap on the other side of the door, that's a +5DC to perception (table Perception p 102 CRB) , which increases the CR of the trap by 1 (see Table 13-3 p 422 CRB).
The reactive perception vs traps is a last ditch chance for a player to not get in trouble. Just as the perception to notice a hidden combatant is there to protect him.
The way it's written is clear, it's just very different from previous ways of doing things. But it's not the only thing that's changed. Acrobatics instead of Jump and Tumble, Stealth instead of Move Silently and Hide... Some things are just more streamlined in Pathfinder.
I'm happy to take my Rogues through entire dungeons taking 20 on every square, but that bogs down the game and is one of the big complaints of other players. This seems like another case of them streamlining something so it works for everyone. A pushy fighter is still going to fall into a trap if he doesn't have the patience to let a rogue roll one perception per room. And you still have to disable the traps. That's not always easy either.