First of all, I don't think those substrs are doing what you think they are.... for instance, the last one probably should be:

# substr EXPR,OFFSET,LEN 
substr($bitmap,14,1) = 1;
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(or maybe 14 should be 13, but I can't tell for sure if you wanted an offset of 14 or position 14)

Second, you can do this type bit flipping w/o the string representation with vec().

-Blake