$s = "\0\1" x 10000;;

cmpthese -1, { 
    A=> q[ $n1 = $s =~ tr[\0][\0] ], 
    B=> q[ $n2 = $s =~ tr[\0][]c  ] 
}; 


     Rate   B   A
B 23522/s  -- -9%
A 25935/s 10%  --

print "$n1 : $n2";;
10000 : 10000