Listing 2: enter-birthday-info.pl, a CGI program written in Perl that
allows users to enter their birthday information.

#!/usr/bin/perl -w

use strict;
use diagnostics;
use CGI;     # see http://www.perl.com/CPAN
use Mysql;

# Create an instance of CGI
my $query = new CGI;

# Output a basic MIME type
print $query->header("text/html");

# Grab information the user gave us, and verify it
my $firstname = $query->param("firstname");
my $lastname = $query->param("lastname");

# Make sure that the user has entered a plausible address
my $email = $query->param("email");
&log_and_die("You entered an invalid e-mail address!") 
 unless ($email =~ /@.*\..*/);

# Gather birthday information
my $birthmonth = $query->param("month");
my $birthday = $query->param("day");
my $birthyear = 
 1900 + (10 * $query->param("yeartens")) + $query->param("yearones");

# Create a date string from that information
my $birthdate = "$birthyear-$birthmonth-$birthday";

# Now that we have the basic information, create an SQL query
my $command = "insert into birthdays ";
$command .= "(firstname, lastname, email, birthdate) ";
$command .= "values ";
$command .= "(\"$firstname\", \"$lastname\", \"$email\", \"$birthdate\")";

# Connect to the database server
my $dbh = Mysql->connect("localhost", "test");

# Send the query for processing
my $sth = $dbh->query($command);

# Check for duplication errors, then for other errors
if ($dbh->errno == 2000)
 {
  &log_and_die("There is already an entry in the database for \"$email\". Try another e-mail address!");
 }
elsif (!defined $sth)
 {
  &log_and_die("MySQL error " . $dbh->errno . " on command \"$command\"<P>" . $dbh->errmsg) unless (defined $sth);
 }

# If there were no errors, then send results to the user
else
 {
  # Return something to the user
  print "<P>Done!</P>\n"; 
 }

# ------------------------------------------------------------
# Useful subroutine for dying with error messages
sub log_and_die
 {
  my $message = shift;
  
  print $query->start_html(-title => "Error!");
  print "<H1>Error</H1>\n";
  print "<P>$message</P>\n";
  print $query->end_html;
  
  die $message;
 }