use Digest::MD5;
...
...
#***********CHECK IF USER ALREADY EXISTS & ITS NOT AN AUTOMATED SCRIPT**********
$select = $dbh->prepare( "SELECT username, date FROM users WHERE username = ? AND date > DATE_SUB(NOW(), INTERVAL 5 MINUTE)" );
$select->execute( $username );

if ( $select->rows ) {
        print h1( {class=>'cyan'}, "Αυτός ο Χρήστης υπάρχει ήδη! Διάλεξε άλλο όνομα χρήστη!" );
        exit 0;
}


#***********ADD NEW USER TO THE DIGEST PASSWORD FILE & TO THE DATABASE**********
my $user  = $username;
my $realm = "You Must Be Registered In Order To Post!";
my $pass  = $password;

open(FILE, ">>/path/to/password/file") or die $!;
     print FILE "$user:$realm:" . Digest::MD5::md5_hex("$user:$realm:$pass") . "\n";
close(FILE);

$select = $dbh->prepare( "INSERT INTO users (username, password, email, date, host) VALUES (?, ?, ?, ?, ?)" );
$select->execute( $username, $password, $email, $date, $host);