You didn't take into account chop()'s destructive behaviour. For 20 of the 21 passes, you're splitting a zero-length string. 
           Rate rev_chop    regex unpack_C unpack_a    split
rev_chop 10.0/s       --      -1%      -3%     -44%     -50%
regex    10.1/s       1%       --      -2%     -44%     -50%
unpack_C 10.3/s       3%       2%       --     -43%     -49%
unpack_a 18.0/s      80%      77%      74%       --     -11%
split    20.2/s     102%      99%      95%      12%       --

[download]
use strict;
use warnings;

use Benchmark qw( cmpthese );

my %tests = (
   split    => q{ my @a = split //, $buf;             },
   regex    => q{ my @a = $buf =~ /./sg;              },
   unpack_C => q{ my @a = map chr, unpack 'C*', $buf; },
   unpack_a => q{ my @a = unpack '(a)*', $buf;        },
   rev_chop => q{ my @a = reverse map chop($buf), 1..length($buf); },
);

$_ = "use strict; use warnings; my \$buf = our \$buffer; $_ 1"
   for values(%tests);

local our $buffer = "abcdef\x00ghik" x 10_000;
cmpthese(-2, \%tests);

[download]

In reply to Re^2: Convert string to array - performance challenge by ikegami
in thread Convert string to array - performance challenge by rtillian

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