That is a fascinating topic. I've run into that error from time to time, and others get it in a different situation from you: when the internal data is already known to be the same as how it should be written (UTF-8).

I think Perl has ratcheted up its features and gradually changed things, and you skipped too many generations in one go.

What version are you using now? The latest 5.14?

I think lots of us would like to discuss or at least see how "wide" data comes to be (you didn't store a funny character directly, right?) and what can control that.

So, it would be especially good if you could post a trivial program that does some manipulation (that's not obviously referring to any wide characters directly), writes it out, and gets that error.