I would actually be willing to let a player trade out perception for two other skills or proficiencies as long as they didn't pick-up perception from a background or a class. I don't really see many players going for that though.
"Customizing a Background
You might want to tweak some of the features of a
background so it better fits your character or the
campaign setting. To customize a background, you can
replace one feature with any other one,
choose any two
skills, and choose a total of two tool proficiencies or
languages from the sample backgrounds. You can either
use the equipment package from your background or
spend coin on gear as described in chapter 5. (If you
spend coin, you can’t also take the equipment package
suggested for your class.) Finally, choose two personality
traits, one ideal, one bond, and one flaw. If you can’t find a
feature that matches your desired background, work with
your DM to create one." - Player's Basic Rules, p. 36 (also PHB)
For the sake of players not getting unintentionally shafted by the DM, I have to point out that this is the default rule--it isn't even listed as a variant. You can create your own background or freely modify them. A background gives you any two skills. That's what it does. The listed backgrounds are only options.
This is even Adventurer's League legal.
"Select the following additional details to help define
your character, as presented in your allowed rules:
• Name
• Alignment
• Ideals
• Bonds
• Flaws
• Background (select one
or create a custom one)
• Faction (optional)" Adventurer's League Players Guide, p. 3