Three tidbits to add to what's already been said:

The (?{...}) and (??{...}) regexp assertions are closures. They capture the lexical scope that existed when the regexp was compiled. The use of a package variable instead of a lexical variable — BrowserUK's solution — thus avoids the problem.

Your code fails if there are newlines in your input. "." won't match a newline without the s modifier.

Aside from unpack, Digest::CRC provides a number of known checksum algorithms in a tested package. Implemented in C (with a Perl fallback) and dedicated to the task, Digest::CRC and unpack should be much faster than your regexp solution.