SOLVED! Thanks to all Details, below.

Just updated to AS 5.24 on W7Pro (This is perl 5, version 24, subversion 0 (v5.24.0) built for MSWin32-x86-multi-thread-64int (with 1 registered patch....)Binary build 2400 [300560] provided by ActiveState ... Built Jul 13 2016 14:32:48)and now am having a script (which behaved as intended under 5.18) popping up the message "Can't start because perl518.dll is missing...." But I can't find any dependency for that .dll nor any other reference to it.

<update> para 1: added perl -v output thru 'patch....)' to more fully report perl -v output. </update>

Script in question is (trivially) modified from Perl Weekly #164 http://perltricks.com/article/114/2014/9/11/Get-a-weather-report-at-the-terminal-with-Perl

#!/usr/bin/env perl
use strict;
use warnings;
use 5.24.0;
use WWW::Wunderground::API;

my $w = new WWW::Wunderground::API(
  location => '12123',
  api_key => 'nxnxnxnx',     # API value concealed; your own is freely
+ avail at Wunderground
  auto_api => 1,
  lang => 'EN',
);

my $home_location = 'Xxxx, XX';

# capture location
print "Enter city or zip code ($home_location): ";
my $location = <>;
chomp $location;

binmode STDOUT, ':utf8'; # for degrees symbol

# print source_time
say "\n WeatherUnderground forecast as of " . scalar localtime . "\n";

# print header
printf "%-10s%-6s%-6s%-8s%-20s\n",
       'Time',
       "TempF",
       "TempC",
       'Rain %',
       'Conditions';

# print hourly
for (@{ $w->hourly })
{
    printf "%8s%6i%6i%8i  %-30s\n",
           $_->{FCTTIME}{civil},
           $_->{temp}{english},
           $_->{temp}{metric},
           $_->{pop},
           $_->{condition};
}
[download]

Execution looks like this:

C:>weather.pl
Enter city or zip code (Xxxxx): Xxxxx

 WeatherUnderground forecast as of Sun Nov  6 11:34:26 2016

Time      TempF TempC Rain %  Conditions
malformed JSON string, neither array, object, number, string or atom, 
+at character offset 0 (before "(end of string)") at C:/Perl/site/lib/
+WWW/Wunderground/API.pm line 113.
[download]

The popup mentioned in para 1 is entitled: "perl.exe - System Error" with text: "The program can't start because perl518.dll is missing from your computer. Try reinstalling the program to fix this problem"

FWIW, 5.18 and the cited .dll are among the items removed in an attempt to have the script look to 5.24. No joy, there. (Running as admin, no ordinary Win7 search ap or util nor regedit finds any file named perl518.dll nor to any other vestige of AS 5.18

<update 1535: perl524.dll is present in C:\Perl\bin -- so does (maybe) something in the API or the modules it uses make perl518.dll a dependency? If so, I failed to find it.</update>

I've also checked the (WWW:Wunderground::) API.pm and have all the modules it uses. The line cited upon execution at the C:> prompt appears below:

$self->raw($result);

    if ($format !~ /(json|xml)/) {
      $self->data->{$action} = $self->raw();
      return $self->raw();
    }

    my $struc = $format eq 'json'   # <--- LN 113 here
      ? decode_json($self->raw)
      : XMLin($self->raw);
[download]

Maybe I'm missing something obvious, but the only things obvious to me are the scabs from pulling my beard out by the roots. Haalp?