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+#!/usr/bin/env perl
+# vim:set ts=4 sw=4 sts=4 et ai wm=0 nu:
+#=============================================================================
+# ch-2.pl Perl Weekly Challenge Task 2 Reversible Numbers
+#=============================================================================
+# Copyright (c) 2023, Bob Lied
+#=============================================================================
+# Write a script to find out all Reversible Numbers below 100.
+# A number is said to be a reversible if sum of the number and its
+# reverse had only odd digits.
+# For example,
+# 36 is reversible number as 36 + 63 = 99 i.e. all digits are odd.
+# 17 is not reversible as 17 + 71 = 88, none of the digits are odd.
+#=============================================================================
+
+use v5.36;
+
+use builtin qw/true false/;
+no warnings "experimental::builtin";
+
+# To be an odd number, it has to be the sum of an even and an odd number.
+# We're considering only two-digit numbers, so that means if the first digit
+# is odd, the second must be even, and vice versa.  Generate the possibilities.
+my @candidate = ( glob("{1,3,5,7,9}{0,2,4,6,8}"), glob("{2,4,6,8}{1,3,5,7,9}") );
+
+say join ",", sort { $a <=> $b }
+                grep { $_ < 100 && allOdd($_ + revNum($_)) } @candidate;
+
+# There are two fairly obvious ways of reversing the number.  We can treat
+# it as a string and reverse the digits, or we can do the math.  The string
+# operations are actually faster.
+sub revNum($n)
+{
+    (my $r = reverse("$n")) =~ s/^0+//;
+    return +($r);
+}
+
+# Checking that all digits are odd.  Again, two possibilities:  do the math
+# one digit at a time, or treat as a string and test each digit.
+sub allOdd($n)
+{
+    my $isOdd = true; # Doesn't handle 0, but 0 is not happening here.
+    while ( $n && $isOdd )
+    {
+        $isOdd &&= ( $n % 10 ) % 2;
+        $n = int($n / 10);
+    }
+    return $isOdd;
+}
+
+sub allOdd_str($n)
+{
+    use List::Util qw/all/;
+    return all { $_ % 2 } split("", $n);
+}