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;
}

##</code><code>##

use Tie::Scalar;
@ISA = (Tie::StdScalar);

sub TIESCALAR { bless { k => $_[1], v => $_[2] }, $_[0] };
sub FETCH { $_[0]{v} }
sub STORE {
   $_[0]{v} != $_[1] and printf "%s: $_[0]{k} logged %s\n",
       scalar localtime, ($_[1] ? "in" : "out");
   $_[0]{v} = $_[1];
}

my (@list, %user) = qw/ list of users to watch/;
tie $user{$_}, main => $_, undef for @list;

while (1) {
    my %curr = map { $_ => 1 } split / /, `users`;
    $user{$_} = $curr{$_} for @list;
    sleep 2;
}