I think it's any constants at all, when the file is reloaded (and we could do it one check per set of files to be reloaded in a request, which will make things a lot less verbose)

You can see it if you, e.g., touch $LJHOME/cgi-bin/DW/InviteCodes.pm; when that is reloaded, you get all the warnings about the constants in there.