checksum calculation (perl?)

I'm using AIX 4.3.2 with ksh and would like to set up an alphanumeric paging script which is compatible with the New Zealand Telecom TAP protocol, which uses a checksum of the message to verify its accuracy.  To achieve this, I would like a script (perferably perl) which performs the following:

1. Take as input, a file which may include printable and "unprintable" characters.

2. Produce as output, a number which is the sum of the ASCII values of all the charaters in the file.

Eg: If the file contains:
  123\n
  A\n
This would add up as follows:
Char   ASCII
  1      49
  2      50
  3      51
  \n     10
  A      65
  \n     10
Total:  235
So, the output should be 235 (decimal).


If you've done that, here's an optional part for extra points:
1. Take the last 12 bits of the above.
2. Convert each group of 4 bits into the ASCII chars 48 ("0") to 63 ("?").
Eg: In the case of a checksum like 12667 (decimal), this converts to binary:
  11 0001 0111 1011
The last 12 bits are:
  0001 0111 1011
Or in decimal:
  1 7 11
And you can just add 48 (decimal) to each of these to get:
  49 55 59
Which can be represented by ASCII chars:
  1 7 ;
So the output should be:
  17;

I will probably "reject" non-perl answers until I give up waiting for perl answers, since perl is my preference.

Thanks.