use strict;
use warnings;
use Benchmark qw(:all) ;

my $m = 1e7;

cmpthese
(
    5,
    {
        Op  => sub { op();  },
	    Ath => sub { ath(); },
    }
);

sub op
{
    my @c;
    my $last = 2;

    for (2 .. $m)
    {
        for (my $a = $_ * 2; $a <= $m; $a += $_)
        {
            $c[$a]++
        }

        $last = $_ if !$c[$_];
    }

    print "$last\n";
}

sub ath
{
    my @d;
    my $last = 2;

    for (my $a = 3; $a <= $m; $a += 2)
    {
        next if $d[$a];

        for (my $b = $a * $a; $b <= $m; $b += $a)
        {
            $d[$b]++
        }

        $last = $a;
    }

    print "$last\n";
}