Why is modulus different in different programming languages?

Question

Perl

print 2 % -18;

-->

-16

Tcl

puts [expr {2 % -18}]

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Answer 1

After dividing a number and a divisor, one of which is negative, you have at least two ways to separate them into a quotient and a remainder, such that quotient * divisor + remainder = number: you can either round the quotient towards negative infinity, or towards zero.

Many languages just choose one.

I can't resist pointing out that Common Lisp offers both.