The explode operator is for arrays, not strings. Use it to split an array into two arrays. The input value must be an array, and the argument is an integer which specifies the split point. It returns an array containing the two resulting arrays.
It seems you have to use the template.ini approach.
After having a look at the code, the explode operator takes an offset and returns 2 arrays. The first array is an array of elements up to the offset and the second is the elements from the offset to the end. This is quite different to what is documented (as Paul has pointed out)
The documentation at http://ez.no/developer/ez_publish_3/documentation/development/libraries/ez_template/operators/array_handling indicates that many of the functions should be able to take both arrays and strings. From looking at the code at lib/eztemplate/classes/eztemplatearrayoperator.php this is not the case.
I think it is an excellent idea to produce documentation before the coding is done. That way it's done and the coders have something to work off and we don't end up in the situation where we are now (limited documentation).
One can only assume that these functions will be completed before the final release.