There's no significant speed difference. For both the scalar and the array, the get to the integer value from the SV or AV, one has to walk a pointer and add an offset.

It may even be that in the scalar case, perl checks IOK flag each time (to see whether it actually has a valid integer value), but that maybe optimized away.

Anyway, there isn't much of a reason to store the length of an array in a seperate variable.