Solution to task 2

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diff --git a/challenge-163/jo-37/perl/ch-2.pl b/challenge-163/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl -s
+
+use v5.16;
+use Test2::V0;
+use List::Util 'reductions';
+use experimental 'signatures';
+
+our $examples;
+
+run_tests() if $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [N...]
+
+-examples
+    run the examples from the challenge
+ 
+N...
+    Calculate the "triangular sum" over the given numbers.
+
+EOS
+
+
+### Input and Output
+
+say triangular_sum(@ARGV);
+
+
+### Implementation
+
+sub triangular_sum (@n) {
+    # Calculate the running sums over the array starting with the second
+    # element until the array becomes a singleton.
+    @n = reductions {$a + $b} @n[1 .. $#n] while @n > 1;
+
+    $n[0];
+}
+
+
+### Examples and tests
+
+sub run_tests {
+
+    # Meeting Slartibartfast, again.
+    is triangular_sum(1, 2, 3, 4, 5), 42, 'example 1';
+    is triangular_sum(1, 3, 5, 7, 9), 70, 'example 2';
+
+    done_testing;
+    exit;
+}