Efficient and compact unix one liner (faster than using -exec on the find command) combining shell and perl, but dangerous if filenames contain newlines or other whitespace (I think some find's have an option to quote the output, but I haven't seen one yet):

find . -type f | xargs perl -pi -e 's/old/new/g'
[download]

See perldoc perlrun for the -p and -i option and modify to '-pi.bak' or somesuch if you want a backup of all files.