As long as we're (micro)optimizing, why even take references to the elements of @_ at all? Just shuffle indices of @_ and grab things out of @_ as they are needed. I guess indexing an array is a tiny bit faster than following a scalar ref.

sub blokhead (@) {
    my @a = (0 .. $#_);
    my $i = @_;
    my $n;
    map+( $n=rand($i--), $_[$a[$n]], $a[$n]=$a[$i] )[ 1 ], @_;
}
[download]

It's clearly not a significant improvement, but does seem to be consistently a tiny bit faster...

On large data:

our @data = map { 'x' x 1000 } 1..1000;
cmpthese -2, { map { $_ => "$_ \@data" } qw/naive listutil buk ikegami
+ blokhead/ };

           Rate    naive  ikegami listutil      buk blokhead
naive    76.1/s       --     -26%     -47%     -88%     -88%
ikegami   103/s      36%       --     -28%     -84%     -84%
listutil  143/s      89%      39%       --     -78%     -78%
buk       638/s     739%     518%     345%       --      -1%
blokhead  643/s     745%     522%     348%       1%       --
[download]

On small data:

our @data = ("xxx") x 1000;
cmpthese -2, { map { $_ => "$_ \@data" } qw/naive listutil buk ikegami
+ blokhead/ };

          Rate    naive listutil  ikegami      buk blokhead
naive    328/s       --     -26%     -44%     -49%     -49%
listutil 445/s      36%       --     -24%     -30%     -31%
ikegami  589/s      80%      32%       --      -8%      -9%
buk      637/s      94%      43%       8%       --      -1%
blokhead 646/s      97%      45%      10%       1%       --
[download]

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