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