Rate ikegami_sort   ikegami         op op_nosort
ikegami_sort   4.68/s           --       -1%       -75%%      -91%
ikegami        4.72/s           1%        --       -75%%      -91%
op             19.1/s         308%      304%         --%      -62%
op_nosort      50.6/s         981%      970%       165%        --
##</code><code>##

                  Rate ikegami_sort   ikegami        op op_nosort
ikegami_sort   0.345/s           --       -1%      -68%      -88%
ikegami        0.348/s           1%        --      -68%      -88%
op              1.08/s         214%      212%        --      -63%
op_nosort       2.90/s         738%      733%      167%        --

##</code><code>##

use Benchmark qw/cmpthese timethese :hireswallclock/;

my $N = 1e6; # N

my @allrefs = map { 1e5 + int rand 9e5 } 1..$N;
my %uni_refs = map { 1e6 + int rand 9e6 => $_ } 1..$N;

cmpthese(timethese(-10, {
    op  => \&op,
    op_nosort => \&op_nosort,
    ikegami => \&ikegami,
    ikegami_sort => \&ikegami,
}));

sub op { 
    my @refs = @allrefs[ sort { $a <=> $b } values %uni_refs ]
}
sub op_nosort { 
    my @refs = @allrefs[ values %uni_refs ]
}
sub ikegami {
    my %keep = map { $_ => 1 } values %uni_refs;
    my @refs = @allrefs[ grep $keep{$_}, 0..$#allrefs ];
}
sub ikegami_sort {
    my %keep = map { $_ => 1 } values %uni_refs;
    my @refs = @allrefs[ sort { $a <=> $b } grep $keep{$_}, 0..$#allrefs ];
}