my @inputArray = (1,2,3,4,5,6,7,8);
my $inputArrayLen = @inputArray;

Even given the recursive context in which the OPed question is couched, I'm a bit surprised that no one has yet mentioned the  $# array operator (if that's the correct terminology), which gives you the maximum index of the array. (Update: See perldata for a discussion of  $# and arrays.)

As others have pointed out, evaluating an array in scalar context as in the
    my $inputArrayLen = @inputArray;
statement yields the number of elements of the array. Using the  $# operator, the statement
    my $maxIndex = $#inputArray;
gives you the highest index of any element in the array.
Leaving aside any considerations of recursion, the idiomatic, "Perlish" way of iterating over an array is something like
    my @array = (...);
    ...
    for my $element (0 .. $#array) {
        do_something_with($element);
        }
Leaving aside any considerations of recursion, a better way of iterating over an array is something like
    my @array = (...);
    ...
    for my $i (0 .. $#array) {
        do_something_with($array[$i]);
        }
and better yet IMHO, the idiomatic, "Perlish" way would be
    for my $element (@array) {
        do_something_with($element);
        }

Of course, there are other ways, such as using the built-in functions map and grep, of iterating over arrays, but those are other stories for other days.

Update: Fixed the brainfart for-loop example.