$ find /path/to/files -name "*.xml" -exec perl -p -i -e 's/this/that/g' {} \;
##</code><code>##

$ find /path/to/files -name "*.txt" -exec -perl -p -i -e 's/these/those/i' {} \;
##</code><code>##

$ find /path/to/files -type f -exec changeIt.pl {} \;