$|=1;

attach_stderr_filter();
while (<>) { 
    print        "To STDOUT: $_"; 
    print STDERR "To STDERR: $_"; 
}

sub attach_stderr_filter {
    my($pid);
    # This is the top secret part :)
    $pid = open(STDERR, '|-');
    defined $pid or die "Cannot fork: $!\n";
    if($pid) {
        return;
    }

    # child, now prints to STDERR.
    while (<STDIN>) { s/foo/bar/; print STDERR "$_"; }
}