broquaint: this appears to fail for an array such as ( -1, -2, 10, -3, 2 ), because on the first iteration (when $_ == 0 ), $_ - 1 is the last element of the array:

my @ary = qw ( a b c 2);
print $ary[-1];
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Please correct (and downvote accordingly :-) if I've missed something

dave