Or get slower lexical variables for $a and $b by providing a prototyping your comparison function and creating my variables to store them in.

From perlfunc(1):

sort SUBNAME LIST
sort BLOCK LIST
sort LIST

  Sorts the LIST and returns the sorted list value.
  ...
  If the subroutine's prototype is "($$)", the ele­
  ments to be compared are passed by reference in
  "@_", as for a normal subroutine.  This is slower
  than unprototyped subroutines, where the elements
  to be compared are passed into the subroutine as
  the package global variables $a and $b (see exam­
  ple below).