#!/usr/bin/env perl -w

use strict;
use IO::Pty;

my $master = new IO::Pty;
my $slave  = $master->slave();

my $pid = fork();
die "Couldn't fork: $!" unless defined $pid;

if ($pid) {
    $master->close_slave();

    while ( !$master->eof ) {
        my $line = $master->getline;
        chomp $line;
        print "TIME: " . localtime() . " OUTPUT: " . $line . "\n";

        if ( $line == 3 or $line == 7 ) {
            my $j = $line++;
            $master->print("$j\n");
        }
    }
    wait();
}
else {
    $master->close();
    $master->make_slave_controlling_terminal();
    $slave->set_raw();

    $slave->print("1\n");
    $slave->print("3\n");
    sleep 1;
    $slave->print("5\n");
    $slave->print("7\n");
    sleep 2;
    $slave->print("9\n");
    $slave->print("11\n");
#   sleep 3;
}

__END__

(1) with $master->print(), and without sleeps:
            $master->print("$j\n");
#   sleep 1;
#   sleep 2;
#   sleep 3;

We have the output:

TIME: Wed Jul  9 11:27:53 2014 OUTPUT: 1
TIME: Wed Jul  9 11:27:53 2014 OUTPUT: 3

(2) Without $master->print(), and without sleeps:
#           $master->print("$j\n");
#   sleep 1;
#   sleep 2;
#   sleep 3;

We have the output:

TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 1
TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 3
TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 5
TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 7
TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 9
TIME: Wed Jul  9 11:31:42 2014 OUTPUT: 11

(3) "sleep 3" at the end makes no difference in terms of output.

(4) with $master->print(), and with "sleep 1":

            $master->print("$j\n");
    sleep 1;
#   sleep 2;
#   sleep 3;

We have the output:

TIME: Wed Jul  9 11:41:05 2014 OUTPUT: 1
TIME: Wed Jul  9 11:41:05 2014 OUTPUT: 3
TIME: Wed Jul  9 11:41:06 2014 OUTPUT: 5
TIME: Wed Jul  9 11:41:06 2014 OUTPUT: 7

(5) with $master->print(), and with "sleep 2":

            $master->print("$j\n");
#   sleep 1;
    sleep 2;
#   sleep 3;

TIME: Wed Jul  9 11:42:52 2014 OUTPUT: 1
TIME: Wed Jul  9 11:42:52 2014 OUTPUT: 3
TIME: Wed Jul  9 11:42:54 2014 OUTPUT: 9
TIME: Wed Jul  9 11:42:54 2014 OUTPUT: 11

(6) with $master->print(), and with both "sleep 1" and "sleep 2":

            $master->print("$j\n");
    sleep 1;
    sleep 2;
#   sleep 3;

TIME: Wed Jul  9 11:43:31 2014 OUTPUT: 1
TIME: Wed Jul  9 11:43:31 2014 OUTPUT: 3
TIME: Wed Jul  9 11:43:32 2014 OUTPUT: 5
TIME: Wed Jul  9 11:43:32 2014 OUTPUT: 7
TIME: Wed Jul  9 11:43:34 2014 OUTPUT: 9
TIME: Wed Jul  9 11:43:34 2014 OUTPUT: 11

Conclusion: When master talks, the slave has to wait, otherwise 
what is written to slave is lost.