Although Evented::Object attempts to maintain compatibility for an extended period of time, a number of exceptions do exist.
Evented::Object 1.* series and above are incompatible with the former versions. Evented::Object 1.8+ is designed to be more thread-friendly and work well in asyncrhonous programs, whereas the previous versions were not suitable for such uses.
The main comptability issue is the arguments passed to the callbacks. In the earlier versions, the evented object was always the first argument of all events, until Evented::Object 0.6 added the ability to pass a parameter to ->attach_event() that would tell Evented::Object to omit the object from the callback's argument list.
The Evented::Object series 1.8+ passes a hash reference $fire instead of the Evented::Object as the first argument. $fire contains information that was formerly held within the object itself, such as event_info, event_return, and event_data. These are now accessible through this new hash reference as $fire->{info}, $fire->{return}, $fire->{data}, etc. The object is now accessible with $fire->{object}. (this has since been changed; see below.)
Events are now stored in the eventedObject.events hash key instead of events, as events was a tad bit too broad and could conflict with other libraries.
In addition to these changes, the ->attach_event() method was deprecated in version 1.8 in favor of the new ->register_callback(); however, it will remain in Evented::Object until at least the late 2.* series.
Version 2.0 breaks things even more because ->on() is now an alias for ->register_callback() rather than the former deprecated ->attach_event().
Version 2.2+ introduces a new class, Evented::Object::EventFire, which provides several methods for fire objects. These methods such as $fire->return and $fire->object replace the former hash keys $fire->{return}, $fire->{object}, etc. The former hash interface is no longer supported and will lead to error.
Version 2.9 removes the long-deprecated ->attach_event() method in favor of the more flexible ->register_callback(). This will break compatibility with any package still making use of ->attach_event().
In order to correspond with other 'Evented' packages, EventedObject was renamed to Evented::Object. All packages making use of EventedObject will need to be modified to use Evented::Object instead. This change was made pre-CPAN.
Long-deprecated callback options may no longer behave as expected in older versions. Specifically, Evented::Object used to try to guess whether it should insert the event fire object and evented object to the callback arguments. Now, it does not try to guess but instead only listens to the explicit options.