That is a very slow way to count characters in a string.

This finds 20 million '0's in a string of 200 million characters in a little over 1/3rd of a second:

C:\test>p1
$s = '0123456789' x 20e6;;
print length $s;;
200000000

say time; 
printf "found %d zeros\n", $s =~ tr[0][]; 
say time;;
1377952426.42676
found 20000000 zeros
1377952426.78727
[download]

Update: The probably reason for your code "flatlining" is because creating an array to hold 200 million individual characters probably requires far more memory (~6.4GB on a 64-bit system) than your system actually has, thus your computer is "thrashing".