I've put $VERSION back into CheckDigits.pm.
I don't know exactly anymore what lead me to remove it from there.
Possibly I did not want to change the file every time without any code
change.
As for $VERSION in each of the *.pm files: why should I want to do this?
At the moment there are 44 *.pm files in the distribution but
CheckDigits.pm is the only one that should be 'use'd in a program.