May I suggest a different approach? Sorry I don't have time to code this. Instead of pre-allocating a key in the hash for each digit, why not create a key for each digit that is found? The number of digits used is then scalar(keys %digits)
I'm not sure if you expect each digit to be different, but if not then you could increment the value to get a count.
Hope this helps, and apologies if I misunderstood.