A module is a loadable segment of code - no more, no less. A class is a specific type of code structure. Therefore a module can contain a class, or several classes, or no classes.

It is typically useful to have one module per class, as that means you can load each class independently easily. But neither one requires the other. A module can be a collection of functions, or even just a segment of procedural code.

Because in object-oriented Perl it is useful (and near-ubiquitous) to have one module per class, the two terms occasionally get used interchangeably in that context.