Find and replace with backup

That command does not run recursively. "*.html" is expanded by the shell to match all files ending in ".html" for the current directory only.

Depending on the total number of files, you could do the same thing recursively like this:

 perl -pi.bak -e 's/arse/tits/' $(find . -iname "*.html")  # for a relatively small number of .html files

or

 find . -iname '*.html' -exec perl -pi.bak -e '{}' \;  # for a larger number of .html files

Actually, I think find has a way to modify -exec's behaviour so that it builds the list of files instead of execing for each match. I can't be bothered to look it up right now.