Replace Perl -w flag with use warnings;

Also delete the useless 2nd shebang line and bump the required Perl
version.

Now the shebang line conforms with Debian policy 4.1.2 to 4.2.0. See:
https://www.debian.org/doc/debian-policy/upgrading-checklist.html#version-4-1-2

Perl v5.8.0 (released in 2002) was chosen because:
  * `use warnings` was added in Perl v5.6.0 (released 2000)
  * The `require` function itself changed in v5.8.0 to support the
    more readable non-numeric version literal syntax.

ddclient

@@ -1,5 +1,4 @@
-#!/usr/bin/perl -w
-#!/usr/local/bin/perl -w
+#!/usr/bin/perl
 ######################################################################
 #
 # DDCLIENT - a Perl client for updating DynDNS information
@@ -19,8 +18,9 @@
 #
 #
 ######################################################################
-require 5.004;
+require v5.8.0;
 use strict;
+use warnings;
 use Getopt::Long;
 use Sys::Hostname;
 use IO::Socket;