foreach is often used with single element as a convenient method of both setting $_ and providing a block in which last can be used.

As for your problem, the code is equivalent to:

 foreach --+   +-- first iteration
           |   |
           v   v
print( (@$res)[0]->[0], "\n");
       ----------
            |
            +-- What $_ contained
[download]

or just:
print "$res->[0][0]\n";