According to the Wikipedia page (linked in the article), Perl uses the floored definition of mod, which is promoted by Knuth, and also used by APL, Fortran and

According to the Wikipedia page (linked in the article), Perl uses the floored definition of mod, which is promoted by Knuth, and also used by APL, Fortran and Common Lisp. https://en.wikipedia.org/wiki/Modulo https://en.wikipedia.org/wiki/Modulo