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Lesson 1

Perl and CGI Module introduction

This module discusses techniques for overcoming some of the inherent limitations of the HTTP protocol. In particular, you will learn:
  1. How to use the state-machine model to keep track of users
  2. How to pass data from page to page using hidden fields
  3. How to use Perl variables in an HTML file
  4. How to use HTTP cookies

In the process, you will build and experiment with the first part of our class project, the Guestbook program.
When you are finished with this module you will have a good understanding of the state-machine model and how to use it to make your programs more interactive for your users.


Each of us has observed physical and other similarities among members of human families. While some of these similarities are due to the common environment these families share, others are inherited, that is, passed on from parent to child as part of the reproductive process. Traits such as eye color and blood type and certain diseases such as red-green color blindness and Huntington's disease are among those known to be heritable. In humans and all other nonviral organisms, heritable traits are encoded and passed on in the form of deoxyribonucleic acid, or DNA for short. The DNA encoding of a single trait is often referred to as a gene. Most human DNA does not encode traits that distinguish one human from another but rather traits we have in common with all other members of the human family. We do share more than 99.9% of our DNA.
DNA consists of long chains of molecules of the modified sugar deoxyribose, to which are joined the nucleotides adenine, cytosine, guanine, and thymine. The scientific significance of these names is minimal. Guanine, for example, is named after the bird guano from which it was first isolated, and we will normally refer to these nucleotides or bases by the letters A, C, G, and T. For computational purposes, a strand of DNA can be represented by a string of As, Cs, Gs, and Ts.