%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%   --