http://www.perlmonks.org?node_id=638682


in reply to Re^2: Need a little help appending lines
in thread Need a little help appending lines

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.)