I had to tweak the 5.6 solution a bit: 

    ike510 => q[
        ( my $out = $in ) =~ s/.\K/\x40/sg;
        print 'ike510',  unpack 'C20', $out if DEBUG;;
    ],
    ike5_6 => q[
        ( my $out = $in ) =~ s/(.)/$1\x40/sg;
        print 'ike5.6',  unpack 'C20', $out if DEBUG;;
    ],

[download]

But the answer to the quesion is 'not well': 

          Rate ike5_6 ike510    buk  shmem  salva   jmac   lanX fletch
ike5_6 0.882/s     --   -30%   -56%   -68%   -73%   -90%   -91%  -100%
ike510  1.26/s    43%     --   -37%   -54%   -61%   -85%   -88%  -100%
buk     1.99/s   126%    58%     --   -28%   -39%   -76%   -81%   -99%
shmem   2.77/s   214%   120%    39%     --   -15%   -67%   -73%   -99%
salva   3.25/s   269%   158%    63%    17%     --   -61%   -69%   -99%
jmac    8.42/s   855%   567%   322%   204%   159%     --   -19%   -97%
lanX    10.4/s  1076%   721%   420%   274%   219%    23%     --   -97%
fletch   309/s 34974% 24395% 15418% 11055%  9407%  3575%  2883%     --

[download]

I suspect that constantly reallocating the results string every 8 insertions does for them.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"I'd rather go naked than blow up my ass"

In reply to Re^2: Interleaving bytes in a string quickly by BrowserUk
in thread Interleaving bytes in a string quickly by BrowserUk

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