C:\>more perl.stdout.fork.pl
use strict;
use warnings;

if (my $pid=fork() ) {
    sleep(1);
    print "parent\n";
} else {
    close STDOUT;
    open STDOUT, '>'. __FILE__.'.temp' or die $!;
    print "print child$/";
    system("echo echo child"); # printed to console, not to .temp
}

C:\>perl perl.stdout.fork.pl
echo child
parent

C:\>more perl.stdout.fork.pl.temp
print child