$ perl -le '
    $, = ", "; 
    @a = (0 .. 20); 
    print @a; 
    @a[2,1] = @a[1,2]; # Magic line
    print @a;'
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
0, 2, 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2