You should give a working snippet to show your problem. Your error says that @cloned_balls is "out-of-scope", which means that where you are trying to set the visibilty on $cloned_balls[0] , it isn't there. When I look at your code, there is a closing } for the the sub balls, before you try to access @cloned_balls. So unless you make @cloned_balls a global, it won't be seen outside the sub balls().

Try printing out @cloned_balls right before you try to set visibilty, and you will see it isn't there. It is only declared withing the sub balls().