Apply ihis patch to WWW::Mechanize.pm to stop the "UTF-8" warnings.
(tested on WWW::Mechanize 1.34)

The following command should do it.  Might need to run it as root.
cd /path/to/perl/lib/ ; patch </path/to/mechanize.patch

More comprehensively you could do:
  perl -e 'foreach $k (@INC) { $p = $k."/WWW/Mechanize.pm"; print "$p\n" if (-f $p); }'
and this would tell you where this file is

If you're brave (or reckless?), you can just do:
sh
for file in `perl -e 'foreach $k (@INC) { $p = $k."/WWW/Mechanize.pm"; print "$p\n" if (-f $p); }'`
do
  sudo patch $file mechanize.patch
done

The loop makes sure it gets all versions you may have installed (i.e. if you
have Perl 5.8.6 and 5.8.8 libraries installed).


--- WWW/Mechanize.pm	2008-08-13 03:14:25.000000000 +0100
+++ WWW/Mechanize.pm	2008-08-13 03:14:49.000000000 +0100
@@ -2140,9 +2140,7 @@

    # Try to decode the content. Undef will be returned if there's nothing to decompress.
    # See docs in HTTP::Message for details. Do we need to expose the options there? 
-    # use charset => 'none' because while we want LWP to handle Content-Encoding for 
-    # the auto-gzipping with Compress::Zlib we don't want it messing with charset
-    my $content = $res->decoded_content( charset => 'none' );
+    my $content = $res->decoded_content;
    $content = $res->content if (not defined $content);

    $content .= _taintedness();