split has an optimisation that occurs under very specific circumstances.

$ perl -MO=Concise,-exec -e'my @a = split //, $buf;'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <0> pushmark s
4  </> pushre(/""/) s/64
5  <#> gvsv[*buf] s
6  <$> const[IV 0] s
7  <@> split[t3] lK
8  <0> pushmark s
9  <0> padav[@a:1,2] lRM*/LVINTRO
a  <2> aassign[t4] vKS
b  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'our @a = split //, $buf;'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  </> pushre(/""/ => @a) s/64
4  <#> gvsv[*buf] s
5  <$> const[IV 0] s
6  <@> split[t5] vK
7  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

In the second program, the assignment is removed and split stores the result into the array itself. It's as if

our @a = split(//, $buf);
[download]

became

split_into(@a, //, $buf);
[download]

Unfortunately, it doesn't currently work for lexicals.

$ perl -MO=Concise,-exec -e'my @a; @a = split //, $buf;'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <0> padav[@a:1,2] vM/LVINTRO
4  <;> nextstate(main 2 -e:1) v:{
5  <0> pushmark s
6  </> pushre(/""/) s/64
7  <#> gvsv[*buf] s
8  <$> const[IV 0] s
9  <@> split[t3] lK
a  <0> pushmark s
b  <0> padav[@a:1,2] lRM*
c  <2> aassign[t4] vKS
d  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

When taking advantage of that optimisation, one gets a solution that's almost twice as fast as the previous solution:

           Rate    regex unpack_C unpack_a    split split_pa
regex    10.4/s       --      -2%     -44%     -50%     -73%
unpack_C 10.6/s       2%       --     -43%     -49%     -73%
unpack_a 18.8/s      79%      76%       --     -10%     -52%
split    20.9/s     100%      97%      12%       --     -46%
split_pa 38.8/s     271%     264%     107%      85%       --
[download]

use strict;
use warnings;

use Benchmark qw( cmpthese );

my %tests = (
   split    => q{ my @a = split //, $buf;             },
   split_pa => q{ local our @a; @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;        },
);

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

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

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