Anonymous Monk:
An exercise of an array of "unknown length" would be to populate an array from a file of unknown lines (thus number of array elements) with each number listed on a line by itself.

In the light of the above statement, let assume, your file with unknown length is given as below __DATA__, consider the codes below:

use warnings;
use strict;

my $sum = 0;
my @nums;    # same as my @nums = ();

while (<DATA>) {
    chomp;
    push @nums, $_;
}

$sum += $_ for @nums;

my $size    = @nums;
my $average = $sum / $size;

print $average, $/;

__DATA__
1
2
3
4
5
6
[download]

NOTE: that @nums = () is assigning an empty list to the variable array @nums, so the array is empty. So, $size will be 0.

You might what to check the following Up:
push, pop, shift, unshift