Some developers have the following convention for library version numbers (and a Perl module is a library to me):

1. Below 1.0 everything is fair game
2. Above 1.0, all incompatible API changes require a new major version number

For a module user this is quite comforting, because it means that upgrades within the same major version number will (ideally) never break your application, and if it does, it's a bug.

For the module author it usually means not to release a 1.0 version until the API is really stable.

So far I adhere to these standards, but that really doesn't mean anything because none of my humble modules have reached 1.0 yet.