The shift and unshift functions work just like pop and push.
However, from the other end of the array.

The shift function takes the element from the bottom of the stack (element 0):

The unshift puts an element on the bottom at element 0.

These functions aren't normally thought of in terms of a stack (although they certainly do work the same); rather they are commonly used to pass parameters to subroutines, including the main function of a Perl program. Their use in that capacity will be covered in detail later in the course.