#!/usr/bin/perl
use strict;
use warnings;
$\ = $/;
my %not_prime   = ();
my $limit = 100; # change this to $ARGV[0] if required
foreach my $x ( 2 .. $limit ) {
    # for all numbers 2 to the maximum required
    next if $not_prime{$x};
    # skip numbers already known not prime
    print "current prime is $x";
    my $y = $x;
    while ( ($x + $y) <= $limit )
    {
        print " * crossing off " . ($x + $y) . ", a multiple of $x";
        $not_prime{ $x + $y } = 1;
        $y += $x;
    }

}

print "Primes between 2 and $limit:";

for ( 2 .. $limit ) {
    unless ( $not_prime{$_} ) {
        print "$_";
    }
}