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 ];
}