Updated: The shell will getcha every time!

I can't really offer much help except I just made a file with the contents above and than ran the command line given (without the redirect):

perl -pe 's/FOO/123/g' blah.txt 
-a = fuhergrg
-b = awerh9w8eh
-c = rgiuhreg
-d = C:\tmp\123.txt

------------
Contents of blah.txt

>more blah.txt 
-a = fuhergrg
-b = awerh9w8eh
-c = rgiuhreg
-d = C:\tmp\FOO.txt
[download]

As you can see, the FOO changed to 123, is this the ACTUAL code you are trying to run, or have you cleansed it for the site?