use Benchmark qw(:all);
use  Math::BigInt;

sub recurse {
  #divide and conquer
  unshift @_, 1 if 2 != @_;
  my ($m, $n) = @_;

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

sub iterate {
   my $result = Math::BigInt->new(1);
   $result *= $_ for 2..shift;
   return $result
}

sub direct {
   my $result =  Math::BigInt->new(shift);
   return $result->bfac();
}

cmpthese(1000, {
        'recurse' => sub { recurse( 100) },
        'iterate' => sub { iterate( 100) },
        'direct' =>  sub { direct(  100) },
    });

##</code><code>##

Benchmark: timing 1000 iterations of direct, iterate, recurse...
    direct:  1 wallclock secs ( 1.68 usr +  0.00 sys =  1.68 CPU) @ 595.24/s (n=1000)
   iterate: 10 wallclock secs ( 9.57 usr +  0.00 sys =  9.57 CPU) @ 104.49/s (n=1000)
   recurse:  9 wallclock secs ( 9.54 usr +  0.00 sys =  9.54 CPU) @ 104.82/s (n=1000)
         Rate iterate recurse  direct
iterate 104/s      --     -0%    -82%
recurse 105/s      0%      --    -82%
direct  595/s    470%    468%      --

##</code><code>##

use Benchmark qw(:all);
use Math::GMP;

sub recurse {
  #divide and conquer
  unshift @_, 1 if 2 != @_;
  my ($m, $n) = @_;

  if ($m < $n) {
    my $k = int($m/2 + $n/2);
    return recurse($m, $k) * recurse($k+1, $n);
  }
  else {
     return new Math::GMP $m;
  }
}

sub iterate {
   my $result = new Math::GMP 1;
   $result *= $_ for 2..shift;
   return $result
}

sub direct {
   my $result = new Math::GMP shift;
   return $result->bfac();
}

cmpthese(10, {
        'recurse' => sub { recurse( 25_000) },
        'iterate' => sub { iterate( 25_000) },
        'direct' =>  sub { direct(  25_000) },
    });

##</code><code>##

Benchmark: timing 10 iterations of direct, iterate, recurse...
    direct:  0 wallclock secs ( 0.24 usr +  0.01 sys =  0.25 CPU) @ 40.00/s (n=10)
            (warning: too few iterations for a reliable count)
   iterate:  8 wallclock secs ( 7.81 usr +  0.00 sys =  7.81 CPU) @  1.28/s (n=10)
   recurse:  8 wallclock secs ( 7.73 usr +  0.00 sys =  7.73 CPU) @  1.29/s (n=10)
          Rate iterate recurse  direct
iterate 1.28/s      --     -1%    -97%
recurse 1.29/s      1%      --    -97%
direct  40.0/s   3024%   2992%      --