Technically you can't do this, but in this particular case you can do it quite easily after the fact:
print_r(array_chunk($arr, 2));
Output:
Array
(
[0] => Array
(
[0] => world=earth
[1] => ;
)
[1] => Array
(
[0] => world!=mars
[1] => ;
)
[2] => Array
(
[0] => world>venus
)
)
See also: array_chunk()