I can only guess what your real file looked like before you posted it thru monasterie's html filters.

> What kind of warning could I insert that would help? It's not a runtime problem;

warn reports the line-numbers saved into the op-tree. °

> That's pretty clear that the Perl 5.26.1 interpreter is providing a buggered line array.

It's not the Perl interpreter, but the debugger which builds the line array. And this with plain Perl code.

UPDATE : I was probably wrong here, seems the population appears when the executable is started with the -d option or if $PERLDB is set to x400 . I thought the actual reading is handled inside perl5db.pl but couldn't identify the sub yet.

> I don't know how to follow up with that, except to upgrade to something newer.

As long nobody else is reproducing this problem, my best guess is that your file is corrupted.

updates

°) see Re^2: Debugger line numbering anomaly (updated)