Perl is the most commonly used language for CGI programming on the World Wide Web .The Common Gateway Interface (CGI) is an essential tool for creating and managing comprehensive web sites. With CGI, you can write scripts that create interactive, user-driven applications.
CGI allows the web server to communicate with other programs that are running on the same machine. For example, with CGI, the web server can invoke an external program, while passing user-specific data to the program (such as what host the user is connecting from, or input the user has supplied through an HTML form). The program then processes that data, and the server passes the program’s response back to the web browser.
Rather than limiting the Web to documents written ahead of time, CGI enables web pages to be created on the fly, based upon the input of users. You can use CGI scripts to create a wide range of applications, from surveys to search tools, from Internet service gateways to quizzes and games. You can increment the number of users who access a document or let them sign an electronic guestbook. You can provide users with all types of information, collect their comments, and respond to them.
For Perl programmers, there are two approaches you can take to CGI. They are:
Programs that handle all CGI interaction directly, without the use of a module such as CGI.pm. While often frowned upon by Perl programmers because it’s more likely to introduce bugs, bypassing the modules has the advantage of avoiding the overhead of CGI.pm for quick, dirty tasks. This chapter explains the concepts of CGI necessary if you intend to write CGI programs from scratch.
CGI.pm is a Perl module designed to facilitate CGI programming. For non-trivial CGI programs, especially ones that need to maintain state over multiple transactions, CGI.pm is indispensable, and is included in the standard Perl distribution as of Perl 5.004. Rather than discuss it in Chapter 8, with the rest of the standard libraries, however, its complexity and importance made it a candidate for its own chapter, Chapter 10.
One performance hit for CGI programs is that the Perl interpreter needs to be started up each and every time a CGI script is called. For improving performance on Apache systems, the mod_perl Apache module embeds the Perl interpreter directly into the server, avoiding the startup overhead. Chapter 11 talks about installing and using mod_perl.