In a true per diem system, you are allocated a specific sum per day (or perhaps half day) and it is yours to spend or not as you wish. Many businesses use this system because it costs them a lot less to process as there are no receipts for meals. You are free to stick a loaf of bread, a jar of peanut butter and a jar of jelly in your bag and live of that if you wish.

If your employer has policy limitations, then you have to ask whether it is worth risking a job on petty fraud. Whether the employer's policy is a good one is not material to the question of whether it's worth the job.