@a = qw( a b c );

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

shift @a;

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