inplace_mode=

Sets the filename extension for inplace editing mode to the given String.
Each file being edited has this value appended to its filename. The
modified file is saved under this new name.

For example:

    $ ruby argf.rb file.txt

    ARGF.inplace_mode = '.bak'
    ARGF.lines do |line|
      print line.sub("foo","bar")
    end