Problem is that I can't seem to shut the server down cleanly if the
client goes down first.  If the server is shut down first, I catch the
Ctrl-C, break out of the while loop, close the socket, and clean up a
log file that I'm writing.

If the client goes down first, I can't seem to catch the Ctrl-C in the
server, the server goes down, and I don't get to close a log file that
I'm writing.

I've registered a sig handler using sigtrap:
use sigtrap qw(handler my_handler normal-signals error-signals);

while ($quit == 0) {
     next unless $sock->recv($msg_in,MAX_MSG_LEN);
     print "$msg_in\n";

sub my_handler {
  ++$quit;

As far as the that goes, you could poll--

my $sel  = new IO::Select;
my $time = 1;

$sel->add($sock);

while ($quit == 0) {
    if ($sel->can_read($time)) {
        $sock->recv($msg_in,MAX_MSG_LEN);
        print "$msg_in\n";
    }