Is there a way to access the previous iteration result/returned value in a loop?

I wouldn't be surprised if your actual use case is more complex, but here's an expression that gives the result you were asking for:

= a!foreach(
 items: 1+enumerate(ri!arrayCount),
 expression: with(
 local!idx: fv!item,
 a!foreach(
 items: 1+enumerate(ri!subarrayCount),
 expression: fv!item + ri!subarrayCount * (local!idx - 1)
 )
 )
)

Having arrayCount=5 and subarrayCount=3, the result is: {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12}, {13, 14, 15}}