12.3 Environment Variables

If you look back to the counter CGI applications in previous chapters, you will see that we saved the counter data in a text file. Some CGI programmers want to avoid using a file, and try to store the information in an environment variable. So they write code that resembles the following:

if ($ENV{'COUNTER'}) {
    $ENV{'COUNTER'}++;
} else {
    $ENV{'COUNTER'} = 1;
} 

To their surprise, however, the counter value is always the same (1, in this case). The point behind this is that you cannot save any environment variables directly from Perl, although it is possible to do so by invoking the shell.

Basically, when a Perl program is started, a child process is created. And the cardinal rule in UNIX is that child processes cannot permanently affect their parent shell.