in reply to Changing every page at once...
Such a script should be fairly straightforward to write, though I doubt it'll be much fun. Have a look at File::Find and the LWP modules.
If your content is fairly well-defined, though, your time might be better spent extracting the content and munging it for use with a templating system, like HTML::Template or Text::Template. Perl.com has an excellent article on templating systems that you might find useful.
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