But if you have to know how much you are storing or transmitting, you need to know what your output file handle is going to do with the data. If the output file handle will be upgrading to utf8, and your data is "\xff123" (4 bytes, 4 characters, in 8-bit encoding), 5 bytes will be written. If the output filehandle downgrades utf8 and you have "\x{ff}123" (4 bytes, 5 characters, in utf8 encoding), you will be writing just 4 bytes. But how long it is in the encoding perl happens to have it stored as is not relevant.