REM foo.bat
@echo OFF
IF "%1" == "process" GOTO processFile
GOTO convertAll

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

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

:end