Another recursive method:

@array = (1,2,3);

print "SUM: ", sum($#array);

sub sum {
  if ($_[0] == 0) {
    return $array[0];
  }
  return $array[$_[0]] + sum($_[0]-1);
}

#note it's not efficient
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This method's shortcoming is that the sub acts only on the global array "@array", and thus can't be easily applied to more than one array. A more elegant and useable recursive method was demonstrated earlier in this thread.

Originally posted as a Categorized Answer.