I'm playing a roleplaying game, too. My guess is you're playing the game like this right now but you're missing what I'm saying. Everyone states how they attack the monster ("I swing my sword at it!"). It's necessary to play the game. As to whether or not there's an attack roll, why would there need to be one if the monster isn't defending itself in any way (for example)? Or what if there's a wall of force in between the character and monster? Do you call for an attack roll then or just narrate the result of the arrow or sword harmlessly bouncing off the invisible barrier? I'm doing the latter because I already know the result. A roll is unnecessary.
This is getting a little bit off-topic of course, so to bring it back to the thread's discussion, how Perception works - or any other attack roll or ability check in the game for that matter - is adjudicated in the same fashion. It all goes back to the basic conversation of the game on page 3 of the Basic Rules: The DM narrates the results of the adventurers actions. If the outcome is uncertain in the DM's eyes, then a roll is called for. In fact, the DMG tells us that there are drawbacks to calling for a roll for every little thing: "...roleplaying can diminish if players feel that their die rolls, rather than their decisions and characterizations, always determine success."