#!/usr/bin/perl -w
use strict;

$| = 1;

my %users;
$users{ $_ } = 0 for split "\n", do{ local $/; <DATA> };

{
    my %in; 
    @in{ split / /, `users` } = ();
    for( keys %users ) {
        my $change = 
             exists $in{$_} && !$users{$_} ? do{ $users{$_}++; 'in'  }
+:
            !exists $in{$_} &&  $users{$_} ? do{ $users{$_}--; 'out' }
+: ();

        $change and print localtime() . " $_ has logged $change\n";
    }
    sleep 2;
    redo;
}

__DATA__
Fred
Wilma
Barney
Betty
[download]