What will happen if you used peroxide on your belly piercing?

Hydrogen peroxide is not something to clean piercings with. They degrade your body's natural clotting and reopen the wound which impede healing and could eventually lead to severe irritation and rejection. Sometimes, due to the wound being open all the time, you can even acquire an infection.

Also, peroxide and alcohol severely weaken acrylic (most barbells have acrylic now) and can cause them to become so brittle that they shatter; shattering could lead to lacerations and infection.