XXX::Simple.pm could mean many things:

• A simpler way of doing XXX, but not necessarily the same way of doing it that XXX.pm does, or with all of the functionality of XXX.pm;
• A simpler interface to XXX.pm, not necessarily with all of the functionality;
• A simple implementation of XXX, where they may not even be an XXX.pm;

Or combinations of the above.

Simply put: a *::Simple module is either a simpler interface, or it does something simply.