He made a small error. He was using %f% where he needed to use %in%. ...or simply %2. Also, I changed foo.bat to %0.

@echo off

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

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

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

:end
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If you really wanted to name the parameter, try the following. It cleans up the environment as a bonus (by spawning another shell where CALL was formerly used).

@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
[download]