# Initial values
my @arr = (1,2,3,  4,5,6,  7,8,9);

# Replace with a set of array refs to 3-element arrays
my $nblk = @arr / 3;
my @a2;
push @a2, [@arr[$_*3, $_*3+1, $_*3+2]] for (0..$nblk-1);

# Sum up each array ref
use List::Util qw(sum);
my @a3 = map { sum @$_ } @a2;