A : pad string with end spaces 
(ABC       )
a: pad string with end null chars 
(ABC       )
C : Pack ASCII values into string (unsigned char) :
C* lets you use var number of chars 
$ustr: 'Hello' 
Unpack that same string -  get ASCII values: 
72:101:108:108:111:
b : Pack bit string (low-to-high) into string (unsigned char) :
$bstr: 'Hello' 
Unpack  -  get original string: 
$str: 0001001010100110001101100011011011110110 
Unpack into hex string (high-to low): 
$hstr: 48656c6c6f 
b : Pack hex string back into string :
original string: Hello