[Euler] Add solution for Euler #28.

Author: shlomif

euler/prob028-shlomif.pl

@@ -0,0 +1,40 @@
+# Solution to Project Euler’s http://projecteuler.net/problem=28
+# by Shlomi Fish
+
+=begin pod
+
+=head1 DESCRIPTION
+
+Starting with the number 1 and moving to the right in a clockwise direction a 5
+by 5 spiral is formed as follows:
+
+21 22 23 24 25
+20  7  8  9 10
+19  6  1  2 11
+18  5  4  3 12
+17 16 15 14 13
+
+It can be verified that the sum of the numbers on the diagonals is 101.
+
+What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed
+in the same way?
+
+=end pod
+
+use v6;
+
+my Int $sum = 0;
+
+my Int $num = 1;
+
+$sum += $num;
+
+for 2, 4 ... 1000 -> $step
+{
+    for 0 .. 3
+    {
+        $num += $step;
+        $sum += $num;
+    }
+}
+print "Sum = $sum\n";