You make an array @array.

Then you make a reference to it using "\" called $array_reference.

Then you pass that reference to a subroutine print_array().

The subroutine dereferences the array (@$reference) and loops over the items in the array, printing them out.

Then you dereference the 4th item (starts at 0) with $$array_reference[3]. (you can also use $array_reference->[3] which I think looks nicer).