Strictly speaking is Modulo only defined for positive integers, excluding 0 for the dividend.

Many languages take incompatible liberties in extending this definition by relying on the same implementation/algorithm, but also allowing floats as input.

The rounding errors theory in your case is very convincing. But you should also check results with negative values and error cases

> Surely, the sane thing to do would be to throw an exception.

NB: JS is particularly reluctant to throw exceptions, it rather follows the "quiet" NaN (= not a number) approach, to silently fail.

IMHO it's impossible to translate this behavior to efficient pure Perl.

Update

I have problems imagining a use case for float modulo operations, except reimplementing division.

Any ideas?