Reading between the lines of the OP and 728564, I think you want something like this:

#!/usr/bin/perl
use strict;
use warnings;
use IO::Socket;
use threads;
use Thread::Queue;
use Mysql;

#ignore child processes to prevent zombies
$SIG{CHLD} = 'IGNORE';

#get the port to bind to or default to 8000
my $port = $ARGV[0] || 8000;

## Queue for commands input from the keyboard
## That will be sent to the currrently connected device
my $qin  = Thread::Queue -> new;

## Queue for the data inbound from the device
my $qout = Thread::Queue -> new;

my $listen_socket = IO::Socket::INET->new(
    LocalPort => $port, Listen => 1,
    Proto => 'tcp', Reuse => 1
) or die "Cant't create a listening socket: $@";

warn "Server ready. Waiting for connections on $port ... \n";

async {
    ## Monitor the keyboard and forward any input
    ## to the currently connected client GPS device
    while( <STDIN> ) {
        chomp;
        $qin->enqueue( $_ );
    }
};

async{
    ## Monitor the queue and do whatever with the inbound data
    while( my $data = $qout->dequeue ) {
        ## Do something with the gps data (like write it to a database
+?)
    }
};

## Spawn a new connection if the device reattaches.
## Old connections clean themselves up automatically
while (my $connection = $listen_socket->accept) {
    threads->create ("read_data", $qin, $qout, $connection)->detach;
    print $connection;
}

sub read_data {
    # accept data from the socket and put it on the queue
    my( $qin, $qout, $socket ) = @_;
    while (<$socket>) {
        print "$_";
        ## Process data from connected devices
        $qout -> enqueue(time." $_");

        ## if there are any commands pending, send them to the GPS dev
+ice
        while( $qin->pending ) {
            my $cmd = $qin->dequeue;
            print $socket $cmd;
        }
    }
    close $socket;
}
[download]

Based on your original above, this is light on error handling. The main changes are:

1. It starts two extra threads (using async()).
   1. One monitors the keyboard and forwards any input (commands) via a second queue, to the current client thread which will in turn forward those to the device.

      This could be an additional listening socket so the command input could be initiated via another script, or even remotely.

   2. The second reads your original queue (which you were doing nothing with), allowing you to process the data asynchronously from it's arrival.

      Going by the use MySql; in the OP, you intend to put it into a DB?

2. Starts a second queue used to pass keyboard input commands to the currently connected device.