%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%   --