Probably not very optimal

gistfile1.txt

# What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
# real	6m17.384s
# user	6m16.172s
# sys	0m1.052
# This is Rakudo version 2016.09-105-g4abc28c built on MoarVM version 2016.09-13-g34c375a
use v6;
my $size = 1001;
my @mat[$size; $size];

init-array(0, $size - 1, $size * $size);
say sum-diagonals;
#say (0..$size - 1).map(-> $idx { @mat[$idx; $idx] + @mat[$idx; $size - 1 - $idx]}).sum - 1;

sub init-array($r, $c, $val) {
  @mat[$r; $c] = $val;
  if $c - 1 >= 0 && @mat[$r; $c - 1] !~~ Int && !($r - 1 >= 0 && @mat[$r - 1; $c] !~~ Int) { # left
    init-array($r, $c - 1, $val - 1);
  }
  elsif $r + 1 < $size && @mat[$r + 1; $c] !~~ Int { # down
    init-array($r + 1, $c, $val - 1);
  }
  elsif $c + 1 < $size && @mat[$r; $c + 1] !~~ Int { # right
    init-array($r, $c + 1, $val - 1);
  }
  elsif $r - 1 >= 0 && @mat[$r - 1; $c] !~~ Int { # up
    init-array($r - 1, $c, $val - 1);
  }
}

sub sum-diagonals {
  my $sum = 0;
  for (0..$size - 1) -> $idx {
    $sum += @mat[$idx; $idx] + @mat[$idx; $size - 1 - $idx];
  }
  $sum - 1; # don't count the '1' in the middle twice
}