If you are willing to look past the warnings in the MIME::Lite documentation, MIME::Lite::HTML will read an HTML file, get/include the images, and transform the src attributes automatically. From documentation:

• Get the file (LWP) if needed
• Parse page to find include images (gif, jpg, flash)
• Attach them to mail with adequat header if asked (default)
• Include external CSS,Javascript file
• Replace relative url with absolute one
• Build the final MIME-Lite object with each part found

I've usually paired it with HTML::FormatText::WithLinks to generate a text version as well

Update: Looks like Email::MIME::CreateHTML is a modern replacement.