I wanted to add this so I don’t have to scour the web every time I need it:

perl -p -i -e 's/oldstring/newstring/g' *

This will replace all occurrences of oldstring with newstring in all of the files in your current directory. Being able to do this quickly and easily is absolutely awesome, but what if you want to do this recursively across the current directory and all directories below that? I’m sure that there are plenty of ways to do this, the one below is the method that seems the easiest:

perl -p -i -e 's/oldstring/newstring/g' `find ./ -name *.html`

If you wanted to be more precise you could replace find with grep and only perform the search and replace on files that you know already contain oldstring, like this:

perl -p -i -e 's/oldstring/newstring/g' `grep -ril oldstring *`