I agree that you have found a bug. I've reduced your testcase even further and the fact that re-opening STDOUT in one fork is affecting the STDOUT in the other fork is clearly demonstrated.

#!perl -slw
use strict;
$|++;

if( my $pid = fork ) {
    print 'Hello from ithread 1';
    sleep 5;
    open STDOUT, '>', 'fork_test.txt' or die $!;
    print STDERR 'STDOUT opened to file';
    waitpid($pid,0);
}
else {
    die 'Couldn\'t fork!' unless defined $pid;
    for( 1..10 ) {
        sleep 1;
        print 'Hello from ithread 2';
    }
}

__END__
P:\>perl58 270224.pl8
Hello from ithread 1
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
STDOUT opened to file

P:\>perl5.6.1 270224.pl8
Hello from ithread 1
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
STDOUT opened to file
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
Hello from ithread 2
[download]

The difference between the 5.6 and the 5.8 versions speaks volumes. As Thelonius said, if all you wanted to do was report the bug, then type "Perlbug" and follow the instructions.

If you are hoping for a work-around then I offer this.

...
else {
    die 'Couldn\'t fork!' unless defined $pid;
    close STDOUT;
    open STDOUT, '>', 'con' or die $!;
     ...
}
[download]

Re-opening STDOUT to 'con', assumes Win32, on unix you'd probably need '/dev/tty' or similar.

Not pretty and it shouldn't be necessary, but it should get you over the hump until you get a proper fix.