The only thing I would add, to yours and BrowserUk's solutions, would be to add an eventloop system in your main program, which can utilize timers to check shared variables on a regular schedule, and do what needs to be done.

This is completely and utterly useless advice. There is absolutely no advantage to using an event loop in the main thread for this.

If there was a requirement to "have the timer in the main program loop do periodic checking of the hash.", that is trivial:

my @maclist = qw (
    7D:6D:62:C6:B4:3D 01:12:79:3E:14:2E 80:27:E4:EA:60:74
    E8:06:88:7F:8C:83 01:26:08:E8:B6:5D 34:15:9E:5D:E6:49
    03:1E:64:CE:25:88 01:0C:29:3F:67:1B 01:22:69:76:5D:F4
    64:B9:E8:10:BF:20 01:1D:FE:D4:7F:E0
);

my @threads;

foreach my $mac (@maclist){
    push @threads, threads->create( \&doit, $mac );
}

while( sleep 1 ) {
   ## check %clients and do things.
}

$_->join for @threads;
[download]

Recommending the addition of huge, monocultural behemoths like POE or GTK2+glib solely to respond to artificial events generated by a timer is asinine. Like hiring a chainsaw to trim your toenails.

You seem to be suffering from the 'When you have a lumphammer' syndrome.