Have a look at IPC::Open3 woes - particularly abstracts(++!) solution using IO::Select to loop through the various filehandles.

The basic idea of the code is as follows:

#!/usr/bin/perl

use strict;
use warnings;

use IPC::Open3;
use IO::Select;
use Symbol;

my $IN  = gensym();
my $OUT = gensym();
my $ERR = gensym();

print "Running...\n";

my $pid = open3($IN, $OUT, $ERR, 'outerr.pl') or die $!;

my $sel = new IO::Select;
$sel->add($ERR,$OUT);

my $data;

while (my @ready = $sel->can_read) {
  foreach my $fh (@ready) {
    read $fh, $data, 8196;

    die "Error from child: $!\n" if ! defined $data;

    if (length $data == 0){
      # No data read - child must be finished writing.
      $sel->remove($fh);
      next;
    } else {
      # read OK:
      print $data;
    }
  }
}

print "Done.\n";
[download]

You should read abstracts' node for a fuller explaination.

I'm not sure if, or how, buffering of STDOUT in your test script will affect things.
A more enlightened monk will fill in the details :-)

On my box (This is perl, v5.8.0 built for i386-linux-thread-multi and IPC::Open3 version 1.0104), I always get STDOUT messages before STDERR, regardless of how the buffering on STDOUT is set in outerr.pl.

BTW, you should consider using strict and warnings.

Hope this helps.

BazB

Update: swapped $ERR and $OUT. Doh. pg++.
Corrected comments about the output order.

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