2 * 3 * 4 * 5 * 6 =
|   |       |   |
|   +-------+   |
+---------------+

4 * 12 * 15 =
|         |
+---------+

12 * 60 =

120
##</code><code>##

sub fact8{
  #divide and conquer without recursion
  my @N = (2 .. shift);
  my @M;
  my $tmp;
  while ($#N){
    while(@N){
      $tmp = pop(@N);
      if (($_ = shift(@N))){push(@M,Math::BigInt->new($tmp)->bmul($_))}
      else {unshift(@M,$tmp)}
    }
    while(@M){
      $tmp = pop(@M);
      if (($_ = shift(@M))){push(@N,Math::BigInt->new($tmp)->bmul($_))}
      else {unshift(@N,$tmp)}
    }
  }
  return @N;
}

perl fact.pl 5000
Method 8 (new func): 28 wallclock secs (28.65 usr +  0.00 sys = 28.65 CPU)
Method 7 (your func): 30 wallclock secs (29.06 usr +  0.00 sys = 29.06 CPU)