[0] Perl> %a = 1..2e6; %b = 1..2e6;;

[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 10000 ], 
    B=>q[ scalar keys %b for 1 .. 10000 ] 
};;
    Rate    A    B
A  804/s   -- -26%
B 1087/s  35%   --