The phrase "perfectly good ___" means that something is good enough for a certain purpose.

We mostly use this phrase to criticize people who want to get something new instead of using something old. For example:

You want a new car? Why? You have a perfectly good vehicle already.

I buy him a perfectly good pair of jeans, and what does he do? He cuts off the bottom of the legs! Can you believe these kids these days?

Don't throw that away. It's still perfectly good.