%a = 1..2e6; %b = 1..2e6;
cmpthese -1, { 
    A=>q[ scalar keys %a,     ++$i for 1 .. 10000 ], 
    B=>q[ scalar keys %b and  ++$i for 1 .. 10000 ],
    C=>q[ my $x = keys %a,    ++$i for 1 .. 10000 ],
    D=>q[ my $x = keys %b and ++$i for 1 .. 10000 ],
}
__END__
   Rate    D    C    A    B
D 280/s   --  -4% -31% -31%
C 290/s   4%   -- -28% -28%
A 404/s  44%  39%   --  -1%
B 406/s  45%  40%   1%   --

##</code><code>##

%a = 1..2e6; %b = ();
cmpthese -1, {
    A=>q[ scalar keys %a, ++$i for 1 .. 10000 ],
    B=>q[ scalar keys %b, ++$i for 1 .. 10000 ],
    C=>q[ my $x = keys %a, ++$i for 1 .. 10000 ],
    D=>q[ my $x = keys %b, ++$i for 1 .. 10000 ],
}
__END__
   Rate    C    D    A    B
C 288/s   --  -2% -30% -31%
D 293/s   2%   -- -28% -29%
A 410/s  43%  40%   --  -1%
B 415/s  44%  41%   1%   --

##</code><code>##

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%    --

##</code><code>##

...
    cmpthese -1, {
        full    => q[scalar keys %full,  ++$i for 1 .. 10000],
        empty   => q[scalar keys %empty, ++$i for 1 .. 10000],
        fake    => q[scalar keys %full,  ++$i for 1 .. 10000],
    }
}
__END__

Length 2 with 22/32items
       Rate  fake empty  full
fake  403/s    --   -2%   -3%
empty 411/s    2%    --   -2%
full  417/s    4%    2%    --

Length 3 with 510/1024items
       Rate  fake  full empty
fake  415/s    --   -1%   -1%
full  418/s    1%    --   -0%
empty 419/s    1%    0%    --

Length 4 with 13961/32768items
       Rate  full empty  fake
full  406/s    --   -2%   -2%
empty 414/s    2%    --    0%
fake  414/s    2%    0%    --

Length 5 with 310721/524288items
       Rate  fake  full empty
fake  406/s    --   -0%   -1%
full  407/s    0%    --   -1%
empty 411/s    1%    1%    --

Length 6 with 8655839/16777216items
       Rate  fake  full empty
fake  400/s    --   -3%   -3%
full  411/s    3%    --   -0%
empty 411/s    3%    0%    --

##</code><code>##

...
    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%    --

##</code><code>##

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

    my $a = 'A';
    $full .= $a while length($full) < $len;

    print "\nLength $len with ", scalar($full), " items\n";
    cmpthese -1, {
        full  => sub { die unless $full },
        empty => sub { die if $empty    },
        fake  => sub { die unless $full },
    }
}
__END__
Length 2 with AA items
            Rate empty  full  fake
empty 29399327/s    --   -5%  -35%
full  31036076/s    6%    --  -31%
fake  44964665/s   53%   45%    --
...