If my perm came out too curly, can I straighten it with a flat iron?

Yes, but be sure to use a flat iron with a variable heat control, such as the Gold N Hot Mini Straightening Iron with Multi-Temp Regulator and do not let the flat iron stay on one section of the hair too long, or you could damage your hair. If your perm is too curly, the best way to fix it is to go back to your stylist for a solution.