A benefit is formal and official, but a perk is an unofficial advantage when doing something. A perk is "I get to leave earlier if the boss is away" A benefit is "I have a company car" Though lots of people use the two words to mean the same thing.

benefit = anything you get: salary, etc perk = extra benefit; use of company car, etc

Perk = a fringe benefit. Example: One of the perks of working with beauty products is that you are sent lots of samples. Benefit= An official payment made or an entitlement available in accordance with a wage agreement, an insurance policy, or a public assistance program. Example: My benefits at work include a company car and a mobile phone.