'A' strips whitespace, etc and 'a' doesn't do that

True, but I think BillKSmith's point was that you were using 'C' instead of 'a'. From pack:

 a  A string with arbitrary binary data, will be null padded.
 A  A text (ASCII) string, will be space padded.
 Z  A null-terminated (ASCIZ) string, will be null padded.
 C  An unsigned char (octet) value.
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Here's the difference:

use Data::Dump;
my $data = "\x01\x02\x03\x20\x00";
dd unpack 'C*', $data;       # prints (1, 2, 3, 32, 0)
dd unpack 'a*', $data;       # prints "\1\2\3 \0"
dd unpack 'A*', $data;       # prints "\1\2\3"
dd unpack 'Z*', $data;       # prints "\1\2\3 "
dd unpack 'Z*', $data."X ";  # prints "\1\2\3 "
dd unpack 'A*', $data."X ";  # prints "\1\2\3 \0X"
dd unpack 'a*', $data."X ";  # prints "\1\2\3 \0X "
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So if you want to retain all of the binary data as a string for further unpacking, the 'a' template is the way to go. 'C' will split that string into its individual bytes and return those values.

Update: Added the second 'A*' example.