#!/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