@echo off

IF "%1" == "process" GOTO processFile
GOTO convertAll

:convertALL
FOR %%f IN (c:\temp\*.txt) DO cmd /c %0 process "%%f"
GOTO end

:processFile
  SET IN=%2
  perl -p -i.bak -e "s/change this/to this/g" %IN%
  del %IN%.bak
GOTO end

:end