system("script.sh >> filename");

##</code><code>##

open my $oldout, ">&STDOUT"
  or die "Can't dup STDOUT: $!";
open STDOUT, ">", $fn
  or die "Can't open $fn: $!";

system $script, @args;

open STDOUT, ">&", $oldout;

##</code><code>##

my $pid = fork;
if (defined $pid and $pid == 0) {
  open STDOUT, ">", $fn or die;
  exec $script, @args;
  exit 1;
}