about the checksum -- you obvously know more than I do about the other end that's sending you the data, but i've always done checksums on the whole record after zeroing the checksum, so you might end up having to pack it back up using the same unpack template...

...but for calculating the checksum itself, you might end up using unpack a third time

from perldoc -f unpack:

For example, the following computes the same
number as the System V sum program:

$checksum = do {
   local $/;  # slurp!
   unpack("%32C*",<>) % 65535;
};
[download]

--twerq