If the extension doesn't contain a "*", then it is appended to the end of the current filename as a suffix. If the extension does contain one or more "*" characters, then each "*" is replaced with the current filename. In Perl terms, you could think of this as: ($backup = $extension) =~ s/\*/$file_name/g; This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix: $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA' Or even to place backup copies of the original files into another directory (provided the directory already exists): $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'