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

our @data = map { 'x' x 1000 } 1..1000;
cmpthese -2, { map { $_ => "$_ \@data" } qw/naive listutil buk ikegami
+ blokhead/ };
...
listutil  143/s      89%      39%       --     -78%     -78%
buk       638/s     739%     518%     345%       --      -1%
blokhead  643/s     745%     522%     348%       1%       --

our @data = ("xxx") x 1000;
cmpthese -2, { map { $_ => "$_ \@data" } qw/naive listutil buk ikegami
+ blokhead/ };
...
ikegami  589/s      80%      32%       --      -8%      -9%
buk      637/s      94%      43%       8%       --      -1%
blokhead 646/s      97%      45%      10%       1%       --