The modulo operator works as I'd expect as long as you don't use integer; as soon as you do, you are at the mercy of your C implementation, which I believe is likely to vary between machines.

On this Linux machine, for example:

  zen% perl -le 'print -1 % 3'
  2
  zen% perl -Minteger -le 'print -1 % 3'
  -1
  zen%
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If you're happy to rely on it, the original request to get the least multiple of $m >= $n can be satisfied by: $n + ((-$n) % $m).

Hugo