Read the commit message for some info.

Okay, read that, and it helped, but I can't say that I understood it all. DB is the debugger package, right? So I guess CORE::caller is setting up ... something ... that helps the debugger do ... something ... and folks who override CORE::GLOBAL::caller have to do the same thing or chaos will ensue? Am I getting that right? :-)

That's because code was added to 5.14 to identify where the bad code is located ($where).

Yeah, I saw that, but, in my case, $where was blank anyway, so I didn't see any point in including that code in retrospect. ;->