The problem in the end appeared to be that perl expects STDOUT to be a tty (my some_application that I was exec()ing was in fact a perl app ... but I wanted a general solution that didn't need to think about that).

When STDOUT isn't a tty, it turns off line buffering. Hence my requirement to snarf STDOUT in real time was thwarted, until I figured the following out.

This code snipped forks and execs an application, using select to wait for available output on the stderr and stdout for the exec()ed application. The streams are not interleaved.

use IO::Pty;
use IO::Select;

my $pty = new IO::Pty;
pipe($readerr, $writeerr);

if (my $pid = fork)
{
    close($writeerr);
    my $select = new IO::Select;
    $select->add($pty);
    $select->add($readerr);
    while (1)
    {
        foreach my $fh ($select->can_read)
        {
            my $buf;
            if (sysread($fh, $buf, 4096))
            {
                print "Read ... $buf ...\n";
            }
        }
    }
}
else
{
    close($readerr);
    $pty->make_slave_controlling_terminal();
    my $slave = $pty->slave();
    close $pty;
    $slave->clone_winsize_from(\*STDIN);
    $slave->set_raw();
    open(STDOUT, ">&" . $slave->fileno);
    open(STDERR, ">&" . $writeerr->fileno);
    close($slave);
    exec("/home/hagus/foo.pl");
}
[download]

--
Ash OS durbatulk, ash OS gimbatul,
Ash OS thrakatulk, agh burzum-ishi krimpatul!
Uzg-Microsoft-ishi amal fauthut burguuli.