I think your template may not be right. You read 20 bytes with your first read, however, the template only unpacks 11 bytes of data.

    print length pack 'H4C3H4b12H4'; # prints 11

    H4 :  4 nybbles =  2 bytes
    C3 :  3 chars   =  3 bytes
    H4 :  4 nybbles =  2 bytes
    b12: 12 bits!!  =  2 bytes
    H4 :  4 nybbles =  2 bytes
                      --
                      11 bytes
[download]

But perhaps you wish to ignore the other 9 bytes in the header.

Unpacking the third field with "H4" gives you a hex string so you should probably use hex() to convert it to an integer. However, if it is a count of the blocks to follow then you could more easily unpack it with "v" or "n" depending on the endianness.

Also in your debug code you can use "H*" as a template for an unknown number of bytes.

--
John.