$a is a special var used by sort.

From the Camel:
"$a PKGThis var is used by the sort function to hold the first of each pair of values to be compared ($b is used to hold the second). (blah...blah...blah) Because it is a global, it is exempt from use strict ; complaints."

It is a special var and should not really be used outside the sort function.

"Nothing is sure but death and taxes" I say combine the two and its death to all taxes!