The perl module math::nocarry implememnts no carry arithmetic which doesn't allow you to carry digits to the next column. for example, if you add 8 and 4, you normally expect the answer to be 12, but that 1 digit is a carry. in no carry arithmetic you can't do that, so the sum of 8 and 4 is just 2. in effect, this is addition modulo 10 in each column. the following example discards all of the carry digits:

1234
+ 5678
------
6802
for multiplication, the result of pair-wise multiplication of digits is the modulo 10 value of their normal, everyday multiplication.