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diff --git a/challenge-136/jo-37/perl/ch-1.pl b/challenge-136/jo-37/perl/ch-1.pl
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+#!/usr/bin/perl -s
+
+use v5.16;
+use Test2::V0;
+use Math::Prime::Util qw(gcd logint);
+use experimental 'signatures';
+
+our ($tests, $examples);
+
+run_tests() if $tests || $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [-tests] [M N ...]
+
+-examples
+    run the examples from the challenge
+ 
+-tests
+    run some tests
+
+M N ...
+    Check if M, N, ... are "friendly".
+
+EOS
+
+
+### Input and Output
+
+say 0 + !!friendly(@ARGV);
+
+
+### Implementation
+
+# I couldn't find any reference to "two friendly".  Maybe Mohammad
+# created this concept?
+# There's no need to restrict the definition to two numbers.
+
+sub friendly (@n) {
+    my ($gcd, $pot) = gcd @n;
+    # Calculate the integer binary logarithm of the GCD together with
+    # its (back-)exponentiation and return "false" for a zero logarithm.
+    logint $gcd, 2, \$pot or return;
+
+    # Check if the GCD is a full power of two.
+    $gcd == $pot;
+}
+
+
+### Examples and tests
+
+sub run_tests {
+    SKIP: {
+        skip "examples" unless $examples;
+
+        ok friendly(8, 24), 'example 1';
+        ok !friendly(26, 39), 'example 2';
+        ok friendly(4, 10), 'example 3';
+    }
+
+    SKIP: {
+        skip "tests" unless $tests;
+
+        ok !friendly(2, 3), 'no common divisor';
+        ok friendly(12, 20, 28), 'three friendly';
+        ok !friendly(12, 36, 60), 'GCD not a power of two';
+	}
+
+    done_testing;
+    exit;
+}