The typical methods for clearing an array are @array = () or undef @array. (These are slightly different in effect; undef will clear (almost) all memory associated with the array but the other method will leave some of it around anticipating that the array is likely to again grow to similar size.)

$#array = -1 can also be used for clearing an array, but I'd consider it unnecessairly confusing.

update: As you noticed $#array=() is equivilent to $#array=0 (since () is a zero-element list update (2): Oops, see anonymous monk's correction below. Note that $#array = () even gives the warning "Use of uninitialized value in scalar assignment" assuming warnings are on.) which makes 0 the last index in @array. This, obviously, does not actually clear the whole array.

Also, note that @array[0] should generally not be used; $array[0] is probably what you want. (Same, of course, goes for @array[1], etc.) There is a subtle difference between the two which may bug you in the future.