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diff --git a/challenge-120/jo-37/perl/ch-2.pl b/challenge-120/jo-37/perl/ch-2.pl
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+++ b/challenge-120/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl -s
+
+use v5.16;
+use Test2::V0;
+use experimental 'signatures';
+
+our ($tests, $examples);
+
+run_tests() if $tests || $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [-tests] [time...]
+
+-examples
+    run the examples from the challenge
+ 
+-tests
+    run some tests
+
+time...
+    calculate the smaller angle between hour and minute hands for the
+    given time(s).
+
+EOS
+
+
+### Input and Output
+
+say clock_angle($_) // "invalid time: $_" for @ARGV;
+
+
+### Implementation
+
+# To find the smaller angle between the hands we need a "double
+# triangular" function in the range (-2m, 2m) with this shape:
+#
+# m     /\      /\
+#      /  \    /  \
+#     /    \  /    \
+# 0  /      \/      \
+#  -2m  -m  0   m   2m
+#
+# which is provided by: f(x) = m - abs(m - abs(x))
+#
+# The minute and hour hands move 6° and 0.5° per minute respectively,
+# resulting in a difference of 5.5° per minute.
+
+sub clock_angle ($time) {
+    # The accepted time format is [H]H:MM.  Distinguishing between
+    # invalid time strings and a zero degree angle.
+    $time =~ /^([01]?\d|2[0-3]):([0-5]\d)$/ || undef and
+        180 - abs 180 - abs $1 % 12 * 30 - 5.5 * $2;
+}
+
+
+### Examples and tests
+
+sub run_tests {
+    SKIP: {
+        skip "examples" unless $examples;
+
+        is clock_angle('03:10'), 35, 'example 1';
+        is clock_angle('04:00'), 120, 'example 2';
+    }
+
+    SKIP: {
+        skip "tests" unless $tests;
+
+        is clock_angle(12345), U(), 'not a valid time';
+        is clock_angle('25:00'), U(), 'not a valid hour';
+        is clock_angle('23:60'), U(), 'not a valid minute';
+        is clock_angle('0:00'), 0, 'omit leading zero';
+        is clock_angle('12:00'), 0, 'zero degrees';
+        is clock_angle('5:00'), 150, 'min to hour';
+        is clock_angle('7:00'), 150, 'hour to min';
+        # The smallest angles besides noon and midnight:
+        is clock_angle($_), 0.5, $_ for (qw(2:11 9:49 14:11 21:49));
+	}
+
+    done_testing;
+    exit;
+}