@a = qw( a b c );

         +---+---+---+---+
    @a = | a | b | c | X |       ( X = spare )
         +---+---+---+---+
           ^       ^
           |       |
           start   end

shift @a;

         +---+---+---+---+
    @a = | X | b | c | X |
         +---+---+---+---+
               ^   ^

##</code><code>##

[ continuing from above ]

push @a, 'd';

         +---+---+---+---+
    @a = | X | b | c | d |
         +---+---+---+---+
               ^       ^

push @a, 'e';

    -> No more space! Re-allocation occurs.
       -> $new_buf_size = $old_buf_size * 2 + 4
       -> Pointers to elements are quickly
          copied to newly allocated buffer.

         +---+---+---+---+
         | X | b | c | d |
         +---+---+---+---+
              /   /   /
             /   /   /
            /   /   /
           v   v   v
         +---+---+---+---+---+---+---+---+---+---+---+---+
    @a = | b | c | d | e | X | X | X | X | X | X | X | X |
         +---+---+---+---+---+---+---+---+---+---+---+---+
           ^           ^