That is a bug in code that tries to track the relative position of the insertion cursor when arrowing up/down through text using a proportional font. Under certain conditions, it comes up with an undefined index value which causes that warning.

FWIW, the code in the current version on CPAN has that particular comparison wrapped in a "no uninitialized" block. In other words, it's already fixed in the current version.

While rooting around in the source though, I noticed that a bug I had reported and gotten fixed several years ago has reemerged. Someone backed out the patch for some reason and now the bug is back.

Sigh.

See this thread.