Interesting.

In any event apparently defined() on the <DATA> file handle is not needed - or at least not any more.

Update: I haven't been able to reproduce this warning on my current Perl 5.24 version. From what I remember the warning was unusually specific in what it said - this was very specific. To my knowledge, I never did actually try Perl code that would cause the problem that was warned about. This was a warning about something that could potentially happen - not an error. This situation did not occur in my source code. I just took Perl at "its word" and adjusted my source code so that this warning went away.

I think of Perl as a living, breathing, biological organism that evolves over time. It is completely possible that this was an extraneous warning about something that didn't really matter. At this point, I don't know. It could be that this warning was just emitted in one version of Perl and corrected in the next version. I've worked with a lot of versions of Perl. I don't have nor would I have saved the exact text of this warning.