Perl solution to Task #1 ("Square Secret Code") of the Perl Weekly Challenge #045

On branch branch-for-challenge-045
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+#!perl
+
+################################################################################
+=comment
+
+Perl Weekly Challenge 045
+=========================
+
+Task #1
+-------
+*Square Secret Code*
+The square secret code mechanism first removes any space from the original
+message. Then it lays down the message in a row of 8 columns. The coded message
+is then obtained by reading down the columns going left to right.
+
+For example, the message is *"The quick brown fox jumps over the lazy dog".*
+
+Then the message would be laid out as below:
+
+  thequick
+  brownfox
+  jumpsove
+  rthelazy
+  dog
+
+The code message would be as below:
+
+  tbjrd hruto eomhg qwpe unsl ifoa covz kxey
+
+Write a script that accepts a message from command line and prints the equiva-
+lent coded message.
+
+=cut
+################################################################################
+
+#--------------------------------------#
+# Copyright © 2020 PerlMonk Athanasius #
+#--------------------------------------#
+
+use strict;
+use warnings;
+use Const::Fast;
+
+const my $COLUMNS => 8;
+
+BEGIN
+{
+    $| = 1;
+    print "\n";
+}
+
+#===============================================================================
+MAIN:
+#===============================================================================
+{
+    @ARGV > 0          or die "Missing command-line message\n";
+
+    my $plain   = join '', @ARGV;
+    my $encoded = encode($plain);
+    my $decoded = decode($encoded);
+
+    print  "Original message:\n>$plain<\n";
+    print "\nEncoded message:\n>$encoded<\n";
+    print "\nDecoded message:\n>$decoded<\n";
+
+    $decoded eq $plain or die "\nCoding error\n";
+}
+
+#-------------------------------------------------------------------------------
+sub encode
+#-------------------------------------------------------------------------------
+{
+    my  ($plain)  = @_;
+    my   @rows;
+    push @rows, substr($plain, 0, $COLUMNS, '') while $plain;
+    my   $encoded = '';
+
+    for my $col (0 .. $COLUMNS - 1)
+    {
+        $encoded .= ' ' if $encoded;
+
+        for my $row (0 .. $#rows)
+        {
+            my $text  = $rows[$row];
+            $encoded .= substr($text, $col, 1) if $col < length $text;
+        }
+    }
+
+    return $encoded;
+}
+
+#-------------------------------------------------------------------------------
+sub decode
+#-------------------------------------------------------------------------------
+{
+    my ($encoded) = @_;
+    my  @rows     = split /\s+/, $encoded;
+    my  $decoded  = '';
+
+    for my $col (0 .. length($rows[0]) - 1)
+    {
+        $decoded .= substr($rows[$_], $col, 1) for 0 .. $#rows;
+    }
+
+    return $decoded;
+}
+
+################################################################################