This will search all files recursively for SEARCH_STRING and replace all occurrences of SEARCH_STRING with REPLACE_STRING throughout each unique file found. It also creates a backup of each modified file so that FILE is backed-up as FILE~ (with a tilde).
grep -R --files-with-matches 'SEARCH_STRING' . | sort | uniq | xargs perl -pi~ -e 's/SEARCH_STRING/REPLACE_STRING/'
