If this is the case ...

It is. Regardless of the OS.

... how come ...

Best guess. The pseudo-code you are posting is a poor reflection of what your real code is actually doing.

I understand that you have good reasons for not posting the real code, so here is a suggestion. Make a copy of the real code and cut it down to just the outer and inner loops you are concerned with. Then substitute the perl 'echo' command from my post above:

my $pid = open2 \*READ, \*WRITE, 'perl -wne "$|++; chomp; print"';
[download]

Run that. It should demonstrate the same behaviours as your real program, though the iterations counts will differ. Once you have verified that it has similar problems, post it here.