What's wrong with making perltidy actually robust enought that we know that nothing has changed.

If you mean either because the file hasn't changed since last it was tidied and perltidy doesn't need to do anything, or because there has been an error and it shouldn't overwrite your original file, then maybe this bit of shell script might prove helpful:

#!/bin/bash

PERLTIDY=/usr/bin/perltidy
PERLTIDYOPTIONS=' -l 123 '
TOUCH=/bin/touch
DIFF=/usr/bin/diff
CP=/bin/cp
CPOPTIONS=' -v '
WC=/usr/bin/wc

if [ $# -gt 0 ]
then
  echo Command line: $0 $@

  for file in $@
  do
    if [ ! -e $file.tdy ]
    then
      TOUCH $file.tdy
    fi
    if [ `$DIFF -q $file $file.tdy | $WC -l` -gt 0 ]
    then
      $PERLTIDY $PERLTIDYOPTIONS $file
      if [ -e $file.ERR ]
      then
        echo Error detected-not replacing
      else
        if [ `$DIFF -q $file $file.tdy | $WC -l` -gt 0 ]
        then
          $CP $CPOPTIONS $file.tdy $file
        else
          echo Skipping replacement of $file - tidied file matches ori
+ginal
        fi
      fi
    else
      echo Skipping $file - perltidy on file not necessary
    fi
  done
else
  echo Usage: $0 filename [filename2]
fi
[download]

Hope that helps.