[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 10000 ], 
    B=>q[ my $x = scalar keys %b  for 1 .. 10000 ] 
};;
   Rate    B    A
B 658/s   -- -19%
A 812/s  24%   --