Check out NestedLoops in Algorithm::Loops. In this case,

use Algorithm::Loops qw( NestedLoops );
use List::Util       qw( reduce sum );

sub mul { return reduce { $a * $b } @_; }

NestedLoops(
   [
      map { [ 0..$n_ra->[$_] ] }
          0..$#$n_ra
   ],
   sub {
      $prob_ra[sum @_] += mul
                          map { $f_raa->[$_]->[$_[$_]] }
                          0..$#_;
   }
);
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I suspect that
$n_ra->[X]
is equal to
$#{$f_raa->[X]}
in which case
map { [ 0..$n_ra->[$_] ] } 0..$#$n_ra
can be replaced with
map { [ 0..$#$_ ] } @$f_raa