The following description from the Camel book (3rd edition, section 28.2) is also helpful:

$a

    [PKG]This variable is used by the sort function to 
hold the first of each pair of values to be compared 
($b is the second of each pair). The package for $a is the 
same one that the sort operator was compiled in, which is 
not necessarily the same as the one its comparison function
 was compiled into. This variable is implicitly localized 
within the sort comparison block. Because it is a global, 
it is exempt from use strict complaints. Because it is an
 alias for the actual array value, you might think you can
 modify it, but you shouldn't. See the sort function.