#!/usr/bin/perl

print "Show primes up to: ";
chomp($max = <>);

@n = (2..$max);
@f = (2..sqrt(@n));

foreach $i (@f) {
        @e = grep {$_ % $i != 0}  @n;
        @n = ();
        @n = @e;
}

print "@n\n";

./sieve.pl
Show primes up to: 21
5 7 11 13 17 19

##</code><code>##

./sieve.pl
Show primes up to: 31
7 11 13 17 19 23 29 31