It's a bad idea to edit a file in-place without a backup, let's just put it that way. Creating a separate output file from your source means your source *is* the backup. So moritz's point, a little less succinctly put, is, "Don't edit a file with important data unless you can undo any changes."
(Or use version control.)
Comment on Re^3: Need a little help appending lines
Precaution against data loss is an entirely different thing, though related.
But that concern doesn't justify marking in-place edits as bad idea, more so generally.
Perl wouldn't have the -i switch if doing in-place edits wasn't a good idea. Generally speaking.
--shmem
_($_=" "x(1<<5)."?\n".q·/)Oo. G°\ /
/\_¯/(q /
---------------------------- \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}