No that's to do with how I usually run perltidy as a filter from emacs and want it to simply print out the tidy'd contents rather than rewrite based on the the input file directly. From the docs:

    -st, --standard-output
        Perltidy must be able to operate on an arbitrarily large number of
        files in a single run, with each output being directed to a
        different output file. Obviously this would conflict with outputting
        to the single standard output device, so a special flag, -st, is
        required to request outputting to the standard output. For example,

          perltidy somefile.pl -st >somefile.new.pl

        This option may only be used if there is just a single input file.
        The default is -nst or --nostandard-output.

    -se, --standard-error-output
        If perltidy detects an error when processing file somefile.pl, its
        default behavior is to write error messages to file somefile.pl.ERR.
        Use -se to cause all error messages to be sent to the standard error
        output stream instead. This directive may be negated with -nse.
        Thus, you may place -se in a .perltidyrc and override it when
        desired with -nse on the command line.