What' even weirder: If I run a sequential verison of the same script, the result is, at least in the last 12104 runs, always correct.

So it's not in the code it must have to do with perl itself.

This is the "serialized" code I used:

#!/usr/bin/perl
use IO::Handle;
use strict;
use warnings;

my $pos= tell *DATA;

  my $x= IO::Handle->new();
  $x->fdopen(fileno(DATA), "r");
  seek $x, $pos, 0;
  print "Start c $pos\n";
  #sleep 4;
  while (<$x>) {
    print "c: $_"; #sleep 1;
  }
  print "stop c\n";
  my $y= IO::Handle->new();
  $y->fdopen(fileno(DATA), "r");
  seek $y, $pos, 0;
  print "Start p $pos\n";
  while (<$y>) {
    print "p: $_"; #sleep 1;
  }
  print "stop p\n";
__DATA__
a
b
c
d
e
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