#!/usr/bin/perl -w
$| = 1;

use strict;
use CGI;
use DBI;
use Time::Piece;

my $min = 5; # number of minutes for timeout

my $dbh = DBI->connect('dbi:mysql:db_name', 'usr', 'pwd', {
    RaiseError => 1, AutoCommit => 1
} );

my $q = CGI->new();
my $track_id = $q->cookie('trackid');

if ( defined($track_id) ) {
    $dbh->do(
        'UPDATE trackids SET lastaccess=NOW() WHERE trackid=?',
        undef, $track_id
    );

    print $q->header();
}
else {
    $track_id = join( '', map {
        chr( int( rand(100) + 1 ) )
    } 1..100 );

    $dbh->do('INSERT INTO trackids VALUES(?, NOW())', undef, $track_id);

    print $q->header( { cookie =>
        $q->cookie( { name => 'trackid', value => $track_id } )
    } );
}

my $time = localtime();
$time -= 60 * $min;
$time = $time->strftime("%Y-%m-%d %H:%M:%S");

$dbh->do(
    'DELETE FROM trackids WHERE lastaccess < ?',
    undef, $time
);

my $rec = $dbh->selectrow_arrayref(
    'SELECT count(trackid) FROM trackids WHERE lastaccess > ?',
    undef, $time
);
$rec = defined($rec) ? $rec->[0] : 0;

print "$rec users online.\n";

__END__

the mysql database:

CREATE TABLE trackids(
    trackid    CHAR(100) NOT NULL,
    lastaccess DATETIME  NOT NULL
);