There is a neat program called ser2net that would allow you to maintain these things as two separate programs. Basically, it would expose the serial port via a tcp socket that program #1 can connect to, and is able to talk bidirectionally just like any socket client.

The second program...the one that just needs read access to what's coming out of the port could do one of two things:

• Continuously watch the output of a "tracefile" that ser2net can provide
• Or, it can also connect (via tcp) to a separate control port, provided by ser2net, send the string "monitor term <port>", and then it will get a streamed copy of everything coming from the serial port.

Use a count (timeout) on the lookfor(), and each time it times out, check a flag to see if there is anything to be sent.

Thanks for all the suggestions. In the end I found this script that uses two non blocking threads and MQTT messages to send and receive info. It does exactly what I needed.

#!/usr/bin/perl

use threads;
use threads::shared;
use JSON;
use WebSphere::MQTT::Client;
use Device::SerialPort;
use Time::HiRes qw(time);
use strict;

# Jeelink port and speed
my $jeelink_port = '/dev/ttyUSB0';

# mqtt configuration
my $mqtt_hostname = 'localhost';
my $mqtt_port = 1883;



# this array is the queu commands to be written to the JeeLink
my @queue : shared = ();

# the thread to subscribes to MQTT messages
sub mqtt {
    my $mqtta  = new WebSphere::MQTT::Client(
        Hostname => $mqtt_hostname,
            Port => $mqtt_port,
        clientid => 'rf12_received_mqtt',
    );

    # connect to mqtt server
    my $res = $mqtta->connect();
    die "Failed to connect: $res\n" if ($res);

    # Subscribe to topic to be forwarded to the JeeLink, multiple topi
+cs can be added here
    $mqtta->subscribe( 'TIME' );

    my @res = ();
      for (;;) {
        @res = $mqtta->receivePub();        
        my $obj = from_json($res[1]);
        my $topic = $res[0];
        
        my $msg = undef;
        if ( $topic eq 'TIME' ) {
            # pack the data into binary
            my $binary = pack("C C C",$obj->{hour},$obj->{minute},$obj
+->{second});

            # convert binary into a command to the Jeelink
            $msg = join(',',unpack("C*",$binary)) . ',0s';
        } 
        # encodes for other topics can be added here 
        
        # push the message to be sent to the JeeLink on the queue
        if ( $msg ) {
            lock(@queue);
            push(@queue,$msg);
        }

    }
}

# the thread that talks to the JeeLink
sub serial {
    # creat an MQTT server reference
    my $mqtt  = new WebSphere::MQTT::Client(
        Hostname => $mqtt_hostname,
            Port => $mqtt_port,
        clientid => 'rf12_sender_mqtt',
    );

    # connect to MQTT server
    my $res = $mqtt->connect();
    die "Failed to connect: $res\n" if ($res);

    # A ping is sent to the MQTT server each five seconds. $ts is used
+ for keeping track of that
    my $ts = time();

    # Set up the serial port
    my $port = Device::SerialPort->new($jeelink_port);
    $port->databits(8);
    $port->baudrate(57600);
    $port->parity("none");
    $port->stopbits(1);

    # start endless loop
    for(;;) {

        # look for data on the serial port
        my $recv = $port->lookfor();

        if ( $recv ) {

            # we're only interested in good messages
            if ( $recv =~ m/^OK / ) {
                my @words = split(' ',$recv);

                # create an empty packet 
                my $obj = {};

                shift(@words); # shift OK
                        $obj->{id} = shift(@words) & 0x1F; # shift and
+ maskthe node id

                my $binary = pack("C*",@words);            
                my $mqtt_topic = undef;            

                # decode a message received from the energy meter
                      if ( $obj->{id} == 0x05 ) {
                            ($obj->{realpower},$obj->{apparentpower},$
+obj->{powerfactor},$obj->{vrms},$obj->{irms},$obj->{netfrequency}) = 
+unpack("f f f f f f",$binary);
                    $mqtt_topic = 'POWERMETER';     
                    }
                # decode a message received from a room node
                elsif ( $obj->{id} == 0x03 ) {
                            ($obj->{light},$obj->{humidity},$obj->{tem
+perature}) = unpack("C C s",$binary);
                            $obj->{moved} = $obj->{humidity} & 1;
                            $obj->{humidity} = $obj->{humidity} >> 1;
                            $obj->{temperature} = $obj->{temperature} 
+& 0x03FF;
                    $mqtt_topic = 'ROOMNODE';
                }

                # send the message as mqtt message encode in JSON     
+       
                if ( $mqtt_topic ) {
                    my $result = $mqtt->publish(to_json($obj),$mqtt_to
+pic);

                    # record the time we sent the last message
                    $ts = time();
                }
            }
    
            }
        
        # if no message was sent each 5 seconds, just send a status me
+ssage to keep us connected to the server
        if ( time() - $ts > 5 ) {
            $mqtt->status();
            $ts = time();
        }

        # check for new packets on the queue
        {
            my $msg = undef;
            {
                lock(@queue);
                if ( scalar(@queue) > 0 ) {
                    $msg = shift(@queue);
                }    
            }

            # write the message to the serial port
            if ( $msg ) {
                $port->write($msg . "\n");
            }
        }
    }
}

# start both threads
my $thr1 = threads->new(\&mqtt);
my $thr2 = threads->new(\&serial);
$thr1->join;
$thr2->join;
[download]