Is a perm suitable in summer?

Not appropriate. It's very hot in summer, so you'd better tie up your hair. If you don't spend a lot of money casually and stay in the barber shop for a few hours, it will all be wasted at the moment you tie your hair!

It is best to have a perm after beginning of autumn. At this time, it will be much cooler sooner or later, and your hair can be well displayed!