Is it simply highlight hidden places thus makes the game easier, or literaly spoof extra stuff into the game?

This is "kinda" important, given you get exp by loot. So a high-PER character will get extra levels and faster leveling, or is it just convenience?

High perception will let you spot invissible things from enemies to hidden passages.
For example: on your first quest you have to clear the outposts. one of them has a locked door. the other outpost on the same map has a hidden trapdoor which leads to a passage to the locked one so you can enter it without needing lockpicking skill. High perception might also let you spot hidden rocks under water which might form a passage to an island with goodies.

But the passage, the island, and the goodies are always there, wether you have the PER, and can grab them, because they are in a fixed location, or this option simply sceese to exist without the PER and with it the exp it means?

Because if the later, then character creation always will start with 10 PER + picking the Snooping perk.

yes, these options are only available if you can detect them. All of them are optional however so it's up to you if you want to invest heavily in PER to grab them. It's perfectly doable without and the underground passage example doesn't give you access to an inaccessable location; it merely allows you to bypass a locked door in case you can't pick the lock.

I kind of like that way of implementing skill checks.

You see my problem is, the exp is based on loot. the loot is based on PER. Therefor low PER might be causing "marignaly possible, but not enjoyable" gameplay, especialy as the game has a notorious infamity of demanding minmaxed characters just to be viable.

when it comes to finding hidden loot, doors or such, i really haven't found perception useful much in finding a LOT of such things. Once in a while you'll find a hidden door that leads to a locker, or a item on the ground, but it isn't a huge deal or benefit when finding loot. Mostly its for increased damage and such with xbows or other items that have increased damage with high perception. Finding hidden loot? not a major thing in the game.