All proposals must be submitted through the Google Summer of Code website. The below is a suggested format, but not required. What's important is that you provide as much information as possible. Technical details don't all need to be figured out, they can be ironed out on the mailing lists after the project starts. Remember, all Summer of Code projects should be large enough for you to work on full time for almost three months. Be sure that you've read the FAQ. Proposals without enough information will be rejected.
Plain-text is best, following something like this format:
Name Email Where can we contact you? Project Title Synopsis A short description. Benefits to the Perl Community Deliverables Quantifiable results e.g. "Improve X modules in ways Y and Z", Project Details A more detailed description. You can't be too detailed. Project Schedule How long do you think the project will take? What are the milestones? Bio Who are you? What makes you the best person to work on this project?