Well, I'm honor-bound to reply that your seeming algorithmic superiority is directly attributable to your pernicious use of bitwise operations. You also have a subtle precedence bug in evilgrep that is fixed in the code below, and invalidates all your results. Furthermore, all your benchmarks are belong to me:

use Benchmark qw(cmpthese);

my @list = (0..100);

cmpthese (-3, {
  c_style  => sub { for (my $i = 0; $i <= $#list; $i += 2) {$list[$i] 
+= 'h1' } },
  slice    => sub { $_ = 'h2' for @list[map {$_ << 1} 0..$#list/2] },
  modulo   => sub { $_ & 1 or $list[$_] = 'h3' for 0..$#list },
  doubler  => sub { $list[$_ << 1] = 'h4' for 0..@list/2 },
  evilgrep => sub { @list[grep{!($_&1)}0..$#list]= ('h5')x @list },
  grep     => sub { @list[grep{!($_&1)}0..$#list]= ('h6')x(@list/2 + 1
+) }
});

### RESULTS ###

evilgrep  7467/s       --     -19%     -40%     -42%     -43%     -65%
grep      9244/s      24%       --     -25%     -28%     -30%     -57%
c_style  12398/s      66%      34%       --      -3%      -6%     -42%
modulo   12811/s      72%      39%       3%       --      -3%     -40%
slice    13211/s      77%      43%       7%       3%       --     -38%
doubler  21289/s     185%     130%      72%      66%      61%       --
[download]

ps. My system is currently underclocked, not to mention having slow timings, 4-way interleaving, and CAS2 disabled. If and when I reboot and tweak, I welcome you to bring it! :P

pps. Assuming of course, my computer boots up...

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print