If you happen to substitute every instances of foo with bar in a specific directory, you can simply use the following shell script.
perl -e "s/foo/bar/g;" -pi $(find /path/to/dir -type f)
You should escape the “/”. Following example will replace foo/foo with barbar
perl -e "s/foo\/foo/barbar/g;" -pi $(find /path/to/dir -type f)