This is really weird…

I removed the sleep commands and printed the $pos in the start message.

Both processes tell the same startposition. Nevertheless I also get non-15 lines of output every now and then. 1536 loops were okay, then the parent didn't read any lines.

My modified 123099.pl code:

#!/usr/bin/perl
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 $pos\n";
  while (<$x>) {
    print "c: $_";
  }
  print "stop c\n";
}
else {
  my $y= IO::Handle->new();
  $y->fdopen(fileno(DATA), "r");
  seek $y, $pos, 0;
  print "Start p $pos\n";
  while (<$y>) {
    print "p: $_";
  }
  print "waiting for child\n";
  wait;
  print "stop p\n";
}
__DATA__
a
b
c
d
e
[download]

And my modified version of your test-bash:

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

Update I've run it several times now and this is the result:

2182:      10
1504:      10
560:      10
14:      10
919:      10
360:      10
908:      10
200:      10
65:      10
108:      10
43:      10
105:      10
296:      10
345:      10
100:      10
1084:      10
1278:      10
224:      10
6:      10
8:      10
125:      10
684:      10
96:      10
8:      10
579:      10
[download]

So after an unpredictable amount of iterations (6 to 2182) the scripts prints 10 instead of 15 lines…