It means that you're outputting a character with code >255 to a filehandle that hasn't been marked as utf8. Perl will assume that you meant for it to be utf8 and do that, but warn you. This is dangerous, because if you're also outputting strings that don't have the utf8 bit set, and they contain chars 128-255, you'll get somthing that is invalid utf8, and just plain wrong in whatever legacy encoding you were using.

(In other words, open FILE, '<:utf8', $whatever). Also, consider that naming your handles FILE and FH gives little information at best, and is confusing at worst.)