Here's seltest.pl:

#!perl -w

use strict;
use IO::Select;
use Socket;
use POSIX;


socketpair(SOCK_READ,SOCK_WRITE,AF_UNIX,SOCK_STREAM,PF_UNSPEC)
    or die "Couldn't create socket pair: $!\n";

print "STDOUT print 1.\n";
if (!defined(my $f = fork()))
{
    die "Fork error: $!\n";
}
elsif (!$f)
{
    # child
    close(SOCK_READ);
    warn "CHILD PID: $$\n";
    close(STDOUT)
    or die "Couldn't close STDOUT: $!\n";
    POSIX::dup2(fileno(SOCK_WRITE),1);
    sleep(100)
    while(1);
}
# parent
close(SOCK_WRITE);

warn "Parent is PID $$\n";

# We can't reliably print to STDOUT after this point (at some point,
# it will start going to the socket)

#sleep(5);

print "STDOUT print 2.\n";
my $sel = IO::Select->new;
$sel->add(\*SOCK_READ)
    or die "Couldn't add pipe to IO::Select: $!\n";

print "STDOUT print 3.\n";
while (1)
{
    print "STDOUT print 4.\n";
    warn "DEBUG: select on ",$sel->count," handles.\n";
    my @ready = $sel->can_read(10);
    warn "Parent DEBUG: ",scalar(@ready)," handles readable.\n";
    print "STDOUT print 5.\n";
    foreach my $rh (@ready)
    {
    my $line=<$rh>;
    warn "Parent read $rh: $line";
    }
    sleep(3);
}
[download]

STDOUT print 1.
Parent is PID 1996
STDOUT print 2.
STDOUT print 3.
STDOUT print 4.
DEBUG: select on 1 handles.
CHILD PID: -1860
Parent DEBUG: 0 handles readable.
DEBUG: select on 1 handles.
Parent DEBUG: 1 handles readable.
Parent read GLOB(0x1b2cee8): STDOUT print 5.
DEBUG: select on 1 handles.
Parent DEBUG: 1 handles readable.
Parent read GLOB(0x1b2cee8): STDOUT print 4.
[download]

As you can see, after a short time the parent starts reading its own input on the handle that should be coming from the child. If you uncomment the sleep(5), then all output from the parent goes to the wrong place. On Unix it behaves as I expected, with all output going to the parent's STDOUT and select nevere returning anything.