Chapter 10. The CGI.pm Module

CGI.pm is a Perl module for creating and parsing CGI forms. It has been distributed with the Perl source kit since 5.004, but you can upgrade your existing CGI.pm installation from CPAN. As of the writing of this edition, the current version of CGI.pm is 2.81.

CGI is an object-oriented module. Don't let the object-oriented nature scare you off, though; CGI.pm is very easy to use, as evidenced by its overwhelming popularity among all levels of Perl programmers. To give you an idea of how easy it is to use CGI.pm, let's take a scenario in which a user fills out and submits a form containing her birthday. Without CGI.pm, the script would have to translate the URL-encoded input by hand (probably using a series of regular expressions) and assign it to a variable. For example, you might try an antiquated parsing method like this:

#!/usr/local/bin/perl 
# CGI script without CGI.pm

$size_of_form_info = $ENV{'CONTENT_LENGTH'};
read ($STDIN, $form_info, $size_of_form_info);

# Split up each pair of key/value pairs
foreach $pair (split (/&/, $form_info)) {

   # For each pair, split into $key and $value variables
   ($key, $value) = split (/=/, $pair);

   # Get rid of the pesky %xx encodings
   $key =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;
   $value =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;

   # Use $key as index for $parameters hash, $value as value
   $parameters{$key} = $value;
}

# Print out the obligatory content type line
print "Content-type: text/plain\n\n";

# Tell the user what they said
print "Your birthday is on " . $parameters{birthday} . ".\n";

Regardless of whether this code actually works, you must admit it's ugly, and completely ignores important details such as dealing with tainted input. With CGI.pm, the script could be written as:

#!/usr/local/bin/perl -w
# CGI script with CGI.pm

use CGI;

my $query = CGI->new( );

my $bday = $query->param("birthday");

print $query->header(-type => 'text/plain');
print $query->p("Your birthday is $bday.");

Even for this tiny program, you can see that CGI.pm can alleviate many of the headaches associated with CGI programming.

As with any Perl module, the first thing you do is call the module with use. You then call the constructor (new( )), creating a new CGI object called $query. Next, get the value of the birthday parameter from the CGI program using the param method. Note that CGI.pm does all the work of determining whether the CGI program is called by the GET or POST methods, and it does all the URL decoding for you. To generate output, use the header method to return the content type header and the p method to generate a paragraph marker <P> tag.

However, this is only the tip of the iceberg as far as what CGI.pm can do for you. There are three basic categories of CGI.pm methods: CGI handling, creating forms, and retrieving environment variables. (A fourth category is creating HTML tags, but we don't cover those in detail.) Table 10-1 lists most of these methods. They are also covered in more detail later in this chapter.

Table 10-1. CGI.pm methods

9.4. CGI Environment Variables		10.2. Importing Method Groups

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