Short one this time.
If you’ve a bignum in Perl, it might be a bit intimidating. For example, how on Earth would you get the third last digit from an enormous number? Convert it to a string first? would that even work? Mod 1000?
Fortunately, bignums are transparently available; once you’ve got one in a variable, you treat it just like any other Perl scalar, so this will work:
$digit = substr($bignum, -3, 1);