If @list doesn't have millions of entries, I'd just copy to a new @tmp array before prior to using your solution.

But in the sense of TIMTOWTDI and with the power of autovivification ... :)

(demo in debugger)

  DB<260> @a= "a" .. "h"

  DB<261> @b=(); $i=0; $C=3

  DB<262> push @{ $b[ $i++/$C ] } , $_ for @a

  DB<263> x @b
0  ARRAY(0x3d7e3b0)
   0  'a'
   1  'b'
   2  'c'
1  ARRAY(0x3d81440)
   0  'd'
   1  'e'
   2  'f'
2  ARRAY(0x3d8e7f8)
   0  'g'
   1  'h'

  DB<264> x @a
0  'a'
1  'b'
2  'c'
3  'd'
4  'e'
5  'f'
6  'g'
7  'h'
  DB<265>
[download]

HTH :)

NB: you haven't been clear about your row $R requirement.

update

it's a poor man's solution for hippo's part() solution in Re: List into two-dimensional array, but since List::MoreUtils isn't core and List::Util doesn't offer it ...