our $ref = { 1 .. 1e6 }; 
our %h = 1 .. 1e6; 
{ 
    my $_ref = { 1 .. 1e6 }; 
    my %_h = 1 .. 1e6; 
    cmpthese -5,{
        a => sub{ exists $_ref->{$_} and 1 for 1 .. 1e6; },
        b => sub{ exists $_h{$_}     and 1 for 1 .. 1e6; },
        c => sub{ exists $ref->{$_} and 1 for 1 .. 1e6; },
        d => sub{ exists $h{$_}     and 1 for 1 .. 1e6; }
    }
};;
    Rate   a   c   d   b
a 3.31/s  -- -2% -3% -4%
c 3.38/s  2%  -- -0% -2%
d 3.39/s  3%  0%  -- -2%
b 3.45/s  4%  2%  2%  --