Benchmark: running substr, vec for at least 60 CPU seconds...
    substr: 61 wallclock secs (62.41 usr +  0.02 sys = 62.42 CPU) @  1.09/s (n=68)
       vec: 58 wallclock secs (60.64 usr +  0.02 sys = 60.66 CPU) @  0.73/s (n=44)
       s/iter    vec substr
vec      1.38     --   -33%
substr  0.918    50%     --

##</code><code>##

use strict;
use warnings;
use Benchmark qw(cmpthese timethese);

my %subs;

###############################################################################

my $UPPER = 1_000_000;

$subs{vec} = sub {
    my $sieve = "";
    GUESS: for my $guess (2 .. $UPPER) {
        next GUESS if vec($sieve,$guess,1);
        #print "$guess\n";
        for (my $mults = $guess * $guess; $mults <= $UPPER; $mults += $guess) {
            vec($sieve,$mults,1) = 1;
        }
    }
};

$subs{substr} = sub {
    my $sieve = '0' x ($UPPER + 1);
    GUESS: for my $guess (2 .. $UPPER) {
        next GUESS if substr($sieve,$guess,1);
        #print "$guess\n";
        for (my $mults = $guess * $guess; $mults <= $UPPER; $mults += $guess) {
            substr($sieve,$mults,1,1);
        }
    }
};

###############################################################################

cmpthese(timethese(-60, \%subs));