sub my_sys {
    my $out = pop;
    my $pid = fork();  die "couldn't fork" unless defined $pid;
    if ($pid) {
        wait;
    } else {
        close STDOUT;
        open STDOUT, ">", $out or die "open '$out' failed: $!";
        exec @_;
        die "exec failed: $!";
    }
}

my_sys("/bin/echo", "foo", "bar",  "std.out");

##</code><code>##

$ ./935901.pl
$ cat std.out
foo bar