It works sometimes . I've run it 10 times and got the following output twice:

Start p
p: a
Start c
stop c
p: b
p: c
p: d
p: e
stop p
[download]

It works sometimes.

As I do an fdopen, I think it should work each time. Infact: I'm just running below code and it's giving me the expected result now the 20th time.

Here is the (slightly modified) code, which I think should work.

use IO::Handle;
use strict;
use warnings;

my $pos= tell *DATA;

my $pid= fork();
die unless defined $pid;
if ($pid == 0) {
  my $x= IO::Handle->new();
  $x->fdopen(fileno(DATA), "r");
  seek $x, $pos, 0;
  print "Start c\n";
  sleep 4;
  while (<$x>) {
    print "c: $_"; sleep 1;
  }
  print "stop c\n";
}
else {
  my $y= IO::Handle->new();
  $y->fdopen(fileno(DATA), "r");
  seek $y, $pos, 0;
  print "Start p\n";
  while (<$y>) {
    print "p: $_"; sleep 1;
  }
  print "waiting for child\n";
  wait;
  print "stop p\n";
}
__DATA__
a
b
c
d
e
[download]

---

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

I've run it 20 times, it returned 15 lines 18 times, but only 10 lines 2 times.

#!/bin/bash
i=1
while : ; do
    n=$( 1230099.pl | wc -l )
    (( n == 15 )) || break
    printf '%d\r' $((i++))
done
echo "$i:$n"
[download]

It stopped after four another iterations with only 10 lines of output.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]