Mike Gossland’s Perl Tutorial Course for Windows

Chapter 6. The CGI Module

Introduction to GET and POST
The CGI Module, CGI.pm
Handling Forms
Handling Form Errors
Handling Fatal Errors

The CGI Module, CGI.pm

The CGI module, introduced quietly in the last example is a superb module for dealing with the CGI interface.

CGI.pm does everything you need to read user input, create forms, handle cookies, handle redirection, and more. It is a very useful module indeed and it is a standard module supplied with your Perl installation.

Let's examine another script just a bit more advanced than the previous one. This next script does everything in the previous script, but it also prints out any input parameters supplied to the script through either GET or POST.

Try saving this script as getpost.pl and running it in your browser window. Or you can run the same script already loaded on my website.

use CGI;

$cgi = new CGI;

for $key ( $cgi->param() ) {
    $input{$key} = $cgi->param($key);

print qq{Content-type: text/html


foreach $key (sort (keys %ENV)) {
    print $key, ' = ', $ENV{$key}, "<br>\n";

for $key ( keys %input ) {
    print $key, ' = ', $input{$key}, "<br>\n";

print qq{<form METHOD=POST><input type="submit" value="Post Request">
<input name="postfield"></form>};

print qq{<form METHOD=GET ><input type="submit" value="Get  Request ">
<input name="getfield" ></form>};

print qq{</body></html>};

The first line "use CGI;" calls the CGI.pm module into your perl script so you can use its features. The very first thing you need to do is to create your brand new shiny $cgi variable, which you do with $cgi = new CGI;. (For those of you who want to know, you have just entered into an object-oriented aspect of Perl, but do not be afraid. It doesn't get particularly complicated.)

As soon as you have defined this new $cgi variable, all kinds of things are available to you. For instance the first thing you see is 

for $key ( $cgi->param() ) {
     $input{$key} = $cgi->param($key);

Now $cgi->param() is a hash of all the (name,value pairs) that were submitted to the form. I like to assign the $cgi->param() hash to the hash "%input" even though it's a duplication of memory, because %input turns out to be easier to work with. If you want the details, email me.

Next we print out the regular Content-type header and get into the html body. Then we work through all the keys in the environment hash, ENV, and print all its (key, value) pairs to the browser.

Lastly, we print out the %input hash. You'll see any values you submit appear here, already neatly parsed into (name,value) pairs. And nice to know, it doesn't matter whether that data came in as the result of a POST or a GET method. Either way, the data are there and accessible in the same way.

Now you've seen how to get at the input fields, let's see a more realistic example in practice.

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