While Digest::MD5 is great, I've used the unpack checksum plenty of times when I've had to test checksums that were already written in data (e.g. lots of data collection hardware use 16-bit checksums in their output). First, I think the 32-bit line should be:
$checksum = unpack("%32C*", $foo) % 65535;

The '%' is just telling unpack to use the special checksum function, I believe the 'C' means to break the input into unsigned chars (it's not for checksum) and the '*' means repeat the 'C' for the rest of the input. You'll want to look at pack for better info on the template syntax the two functions share. According to the manpage, %N works for N-bits, so I have no reason to doubt that your second snippet will do 64-bit checksums, but I can't verify that.

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I'd like to be able to assign to an luser