I beleive it's better to create "independent" subroutines, in that sence of a word that they will not depend on a state of some external global variable. When you'll return to this subroutine after many years and will reuse it, then you have a chance to use it incorrectly.

As a second point, you may accidentially break that external variable somehow and after that something bad will happen, including coredumps (AFAIK some versions of perl will coredump on sort routines when criteria subroutine does not behave well)

Courage, the Cowardly Dog