my %users = map { $u => 0 } qw/ list of users to watch /;

while (1) {
    my %curr = map { $_ => 1 } split / /, `users`;

    for (keys %users) {
        if ( $users{$_} != $curr{$_} ) {
            $users{$_} = $curr{$_};
            printf "%s: $_ has logged %s\n",
                scalar localtime, ($users{$_} ? "in" : "out");
        }
    }
    sleep 2;
}