Seive of Aritosthenes in Perl (print to STDOUT all prime numbers less than or equal to N, in argument -pN).

primes.pl

#!/usr/bin/perl

use Getopt::Std;
use vars '$opt_p';

use strict;
use warnings;

getopts('p:');

# create a list of numbers from 2 to n
# numbers which are not prime will be "marked" with a 0

my %all_numbers = map { $_ => 1 } (2 .. $opt_p);

# all composite numbers <= $opt_p will have one factor <= sqrt( $opt_p )
for my $factor (2 .. sqrt( $opt_p )) {
	# mark all values n that are products of $factor, n <= $opt_p 
	# so, if value not already marked, make a map of its products
	# up to $factor * int($opt_p/$factor)
	# e.g. 100/9 = 11.111.. -> 9*11 = 99 <= 100 < 108 = 9*12
	if($all_numbers{$factor})
	{
		$all_numbers{$_} = 0
			for map { $factor * $_ } ( $factor .. int($opt_p/$factor) );
	}
}

# all_numbers that are not marked false are primes

{
	local $, = ' ';
	print sort { $a <=> $b } grep { $all_numbers{$_} } keys %all_numbers;
}