the letter from the employer isn't 100% needed. its just something extra to have.
but it should state your title, whether you are full time/part time, your salary, and your start date. Ideally it would be on business stationary and signed.
you can find a list of things to bring to the interview here
https://www.visajourney.com/consulates/index.php?ctry=Peru&cty=Lima