...
    cmpthese -1, {
        full    => sub{scalar keys %full,  ++$i },
        empty   => sub{scalar keys %empty, ++$i },
        fake    => sub{scalar keys %full,  ++$i },
    }
}
__END__

Length 2 with 22/32items
           Rate empty  full  fake
empty 4955017/s    --   -4%   -5%
full  5167154/s    4%    --   -1%
fake  5215901/s    5%    1%    --

Length 3 with 510/1024items
           Rate  full empty  fake
full  5038923/s    --   -4%   -5%
empty 5265576/s    4%    --   -1%
fake  5293291/s    5%    1%    --

Length 4 with 13961/32768items
           Rate empty  full  fake
empty 4633858/s    --   -1%   -5%
full  4677165/s    1%    --   -4%
fake  4858803/s    5%    4%    --

Length 5 with 310721/524288items
           Rate  full  fake empty
full  4812084/s    --   -5%   -7%
fake  5041231/s    5%    --   -3%
empty 5193418/s    8%    3%    --

Length 6 with 8655839/16777216items
           Rate  fake  full empty
fake  4689731/s    --   -4%  -16%
full  4906438/s    5%    --  -12%
empty 5604762/s   20%   14%    --