challenge 163, task 2 in Perl

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+#!/usr/bin/env perl
+use strict;
+use warnings;
+use List::Util 'sum';
+
+# You are given a list of positive numbers, @n.
+
+# Write a script to find out the summations as described below.
+
+# Example 1
+# Input: @n = (1, 2, 3, 4, 5)
+# Output: 42
+
+#     1 2 3  4  5
+#       2 5  9 14
+#         5 14 28
+#           14 42
+#              42
+
+# The nth Row starts with the second element of the (n-1)th row.
+# The following element is sum of all elements except first element of previous row.
+# You stop once you have just one element in the row.
+# Example 2
+# Input: @n = (1, 3, 5, 7, 9)
+# Output: 70
+
+#     1 3  5  7  9
+#       3  8 15 24
+#          8 23 47
+#            23 70
+#               70
+
+sub foo {
+    my @rows;
+    $rows[0] = [@_];
+
+    # we'll keep going as long as we have more than 2 elements left
+    while ( @{ $rows[-1] } > 2 ) {
+        push @rows, [
+
+            # we always start with the 2nd element of the previous row
+            $rows[-1]->[1],
+
+            # each following element is the sum of all numbers from the
+            # 2nd element until the position (not index, but position in 
+            # the tabular view above) we are currently filling, of
+            # the previous row.
+            map { sum @{ $rows[-1] }[ 1 .. $_ ] } 2 .. $#{ $rows[-1] }
+        ];
+    }
+
+    # We're supposed to stop and return when we only have one element left,
+    # but it is actually easier to stop at two elements left. The last row
+    # is just the 2nd element of the row before, so we can return that directly.
+    return $rows[-1]->[-1];
+}
+
+use Test::More;
+
+is foo( 1, 2, 3, 4, 5 ), 42;
+is foo( 1, 3, 5, 7, 9 ), 70;
+
+done_testing;