plus the documentation says -b modifies in place (not quite true in this case.)

Truly modifying in place can require lots of memory (since you can't insert into a file, just overwrite), and it fails bad (you lose data if an error occurs halfway through). You should expect "in place" to mean the final result is stored in a file by the same name as the original.

This affects symlinks, and it can also affect permissions. Perhaps a better solution would be to copy the original, then read from the backup and overwrite the original. I know some were interested in changing Perl's -i, but I don't know if that's one of the changes they had in mind. It would slow things down.