#!/full/path/to/perl -wT
use strict;
use CGI qw(some imports);
#... rest of program

##</code><code>##

use CGI::Session;

my $query = CGI->new;
my $session = CGI::Session->new(driver:File", $query, {Directory=>'/tmp'});

# check if user logged in...
if (validate($query->param('user'),$query->param('user'))) {
   $session->param('user') = $query->param('user');
}

my $user_name = $session->param('user'); #get user name from the session (undef unless logged in).

unless ($user_name) {
  # user is not logged in
}
else {
  # user is $user_name
}

# before you print out any content do
print $session->header(); # like $query->header() but with session cookie