#!/usr/bin/perl -w
use strict;

close STDOUT;
open FOO, ">", "/dev/tty" or die $!;
printf STDERR "fileno(FOO): %d\n", fileno(FOO);

open STDOUT, ">", "dummyfile" or die $!;

pipe my $rdr, my $wtr;
printf STDERR "fileno(pipe-r): %d\n", fileno($rdr);
printf STDERR "fileno(pipe-w): %d\n", fileno($wtr);

if (fork) {
    close $wtr;

    my $r = <$rdr>; chomp($r);
    print STDERR "r = <<$r>>\n";

    print FOO "FOO still working\n";
    
} else {  # child
    close $rdr;
  
    printf STDERR "[child] fileno(STDOUT) initially: %d\n", fileno(STDOUT);

    # comment this line out (and edit "&=" below), and you'll see echo will no longer write to the pipe
    open STDOUT, ">&=1";  printf STDERR "[child] fileno(STDOUT) after &=1: %d\n", fileno(STDOUT);

    open STDOUT, ">&".fileno($wtr) or die $!;
    printf STDERR "[child] fileno(STDOUT) finally:   %d\n", fileno(STDOUT);

    exec "/bin/echo", "foobar";
} 

__END__
fileno(FOO): 1
fileno(pipe-r): 4
fileno(pipe-w): 6
[child] fileno(STDOUT) initially: 3
[child] fileno(STDOUT) after &=1: 1
[child] fileno(STDOUT) finally:   1
r = <<foobar>>
FOO still working