My point of view is that it should be the job of the module writer to guess every possible situation of when a object. And this applies fine was a simple object like the Employee or Person often used in OO tuts. But it begins to break down when you get objects which could benefit from higher-grain locks. My approach would not to include locking in the core modules since this increases overhead in the large number of applications with threads, but to instead use an adaptor or subclass to wrap locks around each record and to manage them.
You make a good point that by encapsulation you shouldn't need to know about locking, but surely the core algorithms for mapping a file to an array don't need to know about thread locking? This would be best served by adding another layer on top of the existing class.
integral, resident of freenode's #perl