use Device::SerialPort;

my $port = Device::SerialPort->new( "/dev/ttyS0" );
$port->baudrate(9600);
$port->databits(8);
$port->stopbits(1);
$port->parity("none");

my $string = '';
my $more = 1;
while($more) {
	my $pass = $port->write('S$');
	my $end_seen = 0;
	while ( not $end_seen ) {
		local $_ = $port->input;
		if ( $_ ne '' ) {
			if ( m/S:/ ) { # string start
				$string = $_;
			}
			elsif ( m/:$/ ) { # string end
				$string .= $_;
				print $string; # deal with the final string
				$end_seen=1;
				$more=0;
			}
			else {
				$string .= $_;
			}
		}
	}
}