$sum = 0; 
$sum += $_ for unpack 'C*', "This is a string to change to binary"; 
print $sum;;
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Or

print unpack '%32C*', "This is a string to change to binary";;
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But the result will be modulo 2**32 (assuming it ever gets that high; which won;t happen until your strings are at least 16 million characters long.

If that was a concern, on a perl with 64-bit ints you can do:

print unpack '%64C*', "This is a string to change to binary";;
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Which will handle strings to at least 72057594037927936 bytes, which should be enough to be going on with :)