The code given is a fragment. We don't see how $hash0 was made. Can you post a minimal piece of code that will generate the same error that you are getting, when we run it. The above fragment runs just fine and does nothing. If I add strictures and warnings it fails as $hash0 is not defined. Often when going through the process of creating a minimal failing program, you will find enlightenment.

Cheers,
R.