You can checksum the whole volume by not mounting it and just reading the /dev/whatever raw contents as a stream of bytes. This should be much faster than traversing a file structure.

However, Digest::MD5 is going to be much slower than cksum. It does more work. If you want to generate a CRC in Perl rather than by calling cksum, you might want to look into Compress::Zlib, String::CRC, or String::CRC32, which all provide a CRC function.