Basically this looks good. Here's a few thoughts and suggestions:

• Your if-elsif has no else. In this case, handling missing or unknown source (as alluded to by bart) would be appropriate.
• Consider making the source types case-insensitive.
• Do you need to pass %args to _init()?
• You may find it useful to have an _init() in My which calls $self->SUPER::_init().
• Is there a reason to return $self in list context?