Is your $excel object valid at the time of the error? Is $_[2] what you expect it to be? If you change the order of the files, do you get the same error on the same file? Or do you get the same error on the same iteration? Please provide more information about how you've attempted to find the problem and what results your efforts produced.

Also, please use <code></code> tags around your code, so that we can read it.