If you use i.e. the String::CRC32 module, just use the incremental form $crc=crc32($additionalString,$crc) starting with $crc=0;. Finally, $crc is a 32bit unsigned integer value that can be compared. Maybe you need a conversion into a hex-notation or something beforehand (Edit: I mean before comparison, if you have to compare against some string format.): $clear=sprintf("%X",$crc); or $clear=uc(unpack("H8", pack("N",$crc)));