use strict;
use warnings;
use Benchmark qw(cmpthese);

for my $len (2..6) {
    our (%full, %empty);

    my $a = 'A';
    $full{$a++} = $a while length($a) < $len;

    print "\nLength $len with ", scalar(%full), "items\n";
    cmpthese -1, {
        full    => q[scalar keys %full,  ++$i for 1 .. 10000],
        empty   => q[scalar keys %empty, ++$i for 1 .. 10000],
    }
}
__END__

Length 2 with 22/32items
       Rate empty  full
empty 350/s    --  -15%
full  411/s   17%    --

Length 3 with 510/1024items
       Rate  full empty
full  406/s    --   -2%
empty 415/s    2%    --

Length 4 with 13961/32768items
       Rate  full empty
full  409/s    --   -1%
empty 414/s    1%    --

Length 5 with 310721/524288items
       Rate empty  full
empty 414/s    --    0%
full  414/s    0%    --

Length 6 with 8655839/16777216items
       Rate empty  full
empty 407/s    --   -4%
full  426/s    5%    --