The meaning of perk up is "to wake up, sleepy head".

This cayenne pepper ought to perk up that salad.

If you mean how can you level up your perks in killing floor?

Check your perk list and on a window it'll show the requirements to do in order to level up a perk...

As far as i know you need a specail perk. If you are talking about online. It is called the money perk.

Cowboys loved a colorful phrase! This meant lively, brisk, holding up your head. A good horse was perk.