This is yet another example (there have been several of these over the past week or two, huh?) of exercising ambiguities in perl's grammar. The expression you've written could be taken to mean a couple of different things:

• either a sort being called on the return value of a function called on a list
• or a sort being called on a list, with a particular function being used as the comparitor in the sort (that is, a function which can be called by sort, to tell it how any two items in the list compare to one another)

The perl parser has all sorts of crazy rules for determining what it thinks you meant when it runs accross these ambiguous parses. My personal favorite way to disambiguate is with the no-op unary plus operator.