Wow, are you gonna get some answers... Have a look at some of these threads. There are plenty of ideas, suggestions and lessons.

• •web site design, or lack thereof -- essential
• Perl/CGI Security Question
• Securing Passwords
• Essential CGI Security Practices
• CGI Security
• CGI Password/Login and Security
• CGI Password

There are more. I'm sure I've missed two or three excellent threads, but if you SuperSearch you should find plenty to be enlightened by. It isn't just the client you should worry about...