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find . -type f -name "*.html" ¦ xargs perl -pi -e 's/%24/&/g';
Just type this on your unix console shell command prompt and you are done.
The character before the xargs is a vertical bar (unix pipe), just in case this forum software should mangle this character.
Caution: as this command will overwrite your files, test it first with some test files in a test directory and check the results, before you mangle your production files.
Disclaimer: although this command worked for me very well several times, you are using it at own risk.
Hope this helps.