Method 6: 14 wallclock secs (14.31 usr +  0.01 sys = 14.32 CPU)
Method 7: 15 wallclock secs (14.28 usr +  0.01 sys = 14.29 CPU)
Method 8: 14 wallclock secs (14.25 usr +  0.01 sys = 14.26 CPU)

##</code><code>##

Method 6: 14 wallclock secs (14.08 usr +  0.02 sys = 14.10 CPU)
Method 7: 14 wallclock secs (14.14 usr +  0.01 sys = 14.15 CPU)
Method 8: 15 wallclock secs (14.12 usr +  0.01 sys = 14.13 CPU)

##</code><code>##

sub fact6b{
  my ($m, $n) = @_; 

  if ($m < $n) {
    my $k = int($m/2 + $n/2);
    return my $x = fact6b($m, $k) * fact6b($k+1,$n);
  }
  else {
    return Math::BigInt->new($m);
  }
}