If you're on a unix machine, the most fun way to do it would be with something like:

find . -type f -exec perl -pi -e "s/string x/string y/g" {} \;
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This will find all files, starting in the current directory, and do the replacement intended. If you do not have access to the find command, you're probably going to want to look at File::Find, as it's the standard perl module to handle recursively finding files, and letting you perform some editting on each.