it might not be prudent to change the prototype on this low-level call

Note you're not changing it, just telling Perl to ignore it for that one call, meaning you get Perl's standard behavior of flattening the argument list. But indeed it means that you need to be certain that your subroutine will always return the offset and length; the workaround that choroba showed at least gives you the oppertunity to check that. And LanX's suggestion to wrap it in a sub is also good.