[0] Perl> %a = 1..1e6;;
[0] Perl> %b = ();;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
    Rate    A    B
A 8354/s   -- -15%
B 9845/s  18%   --

[0] Perl> %b = (1..2e6);;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   A   B
A 821/s  -- -0%
B 824/s  0%  --

[0] Perl> %a = ();;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate    B    A
B 815/s   -- -18%
A 998/s  23%   --

[0] Perl> %a = (1,2);;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   A   B
A 843/s  -- -1%
B 849/s  1%  --

[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   B   A
B 804/s  -- -3%
A 831/s  3%  --