Didn't see any warning, but the code quickly evolved to this:

  my $hr = $tvdb->getSeriesBanners($title, 'season', 'seasonwide');
  my $rating = 0;
  my $bwurl;
  my @keys = keys %$hr;
  foreach my $id (@keys) {
    my $hr2 = $hr->{$id};
    if (($hr2->{Season} == $seasonnum) && ($hr2->{Rating} > $rating)) 
+{
      $rating = $hr2->{Rating};
      $bwurl = $hr2->{BannerPath};
    }
  }
  if ($bwurl) {
    debug(1, "Using highest rated Series Banner\n");
    return "http://thetvdb.com/banners/_cache/$bwurl";
  }
[download]

In which I now see a bug: If the only banner(s) available have a zero Rating, I end up with nothing. I'll change it to -1.

The Rating is a 0..10 scale AFAIK (Thetvdb.com via TVDB::API), and I always use strict; use warnings;