That particular error can occur when a non-numeric number is being used with the operator. (I was able to recreate this error with your code, but haven't been able to replicate it yet.) When using a string you want to use .= instead.

Without seeing the form it's hard to tell what should be processed in the loops.

I think what might be happening is the post data is being overwritten by one of the lines using the $perms variable. This really depends on how your environment is setup, and how everything is scripted.

Let me know if this helps. If you can provide a more information on what the form is submitting I could probably make more sense of this.