open(my $in, '/etc/passwd');

my $flags = fcntl($in, F_GETFD, 0);
fcntl($in, F_SETFD, $flags & (~FD_CLOEXEC)) or die "Can't set flags: $!\n";

if (!fork()) {
        exec("grep", "root", "/proc/self/fd/" . fileno($in));
}
close($in);