What part of this baffles you? The fact that they are omitting what to split? The fact that they are not providing a delimiter? Or how they are assigning the results of split?

For the first, It's splitting what's in $_. If no delimiter is specified, it will split on whitespace. And as far as the assignment of the results, it's taking the first 2 elements of the return values and assigning them to $a and $b. Then, all the rest of the return values are put into the array @rest. This is the documentation for split

Hope this helps..
Rich