Perl will handle array size (memory) as needed while populating an array. The "unknown length" part becomes crucial when you have to know the number of array elements beforehand if you were to write a C, C++, or Java program to fill an array. (In latter two languages there are other data structures -- vector, set, etc. -- to avoid the need to know the number of elements beforehand).
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.