Return split results as arrayref instead of list

Andy, sorry but I just noticed I made an inadvertent change in that 5.18.0 split fit.

In 0caac1347e2e3f2fc290f0a4ffc0281b5df68abb I accidentally changed the return behavior from arrayref to list, which worked fine, but could be slower when large lists are returned and in any case was an unintended change.