use strict;
use warnings 'all';

sub factorial_r {
    my ($n) = @_;
    
    return $n if ($n < 2);
    return $n * factorial_r($n-1);
}

sub factorial_i {
    my ($n) = @_;
    
    my $i = $n - 1;
    $n *= ($n - $i--) while $i > 1;
    return $n;
}

sub factorial_r_extra {
    my ($n) = @_;
    my $x = "blah" x 100;
    return $n if ($n < 2);
    return $n * factorial_r_extra($n-1);
}

sub factorial_i_extra {
    my ($n) = @_;
    my $x = "blah" x 100;
    my $i = $n - 1;
    $n *= ($n - $i--) while $i > 1;
    return $n;
}

use Benchmark qw(timethese);
timethese(100000, {
    first  => sub { factorial_r(50) },
    second => sub { factorial_i(50) },
});

##</code><code>##

Benchmark: timing 100000 iterations of factorial_i, factorial_i_extra, factorial_r, factorial_r_extra...
factorial_i:  5 wallclock secs ( 4.94 usr +  0.00 sys =  4.94 CPU) @ 20251.11/s (n=100000)
factorial_i_extra:  6 wallclock secs ( 5.67 usr +  0.00 sys =  5.67 CPU) @ 17633.57/s (n=100000)
factorial_r:  6 wallclock secs ( 6.44 usr +  0.01 sys =  6.45 CPU) @ 15496.67/s (n=100000)
factorial_r_extra: 13 wallclock secs (11.98 usr +  0.00 sys = 11.98 CPU) @ 8344.46/s (n=100000)