my $downloader = eval { new Downloader() } or die($@);

print $downloader; ## This will be a different instance 

my $thread1 = threads->new(
   sub {

      print $downloader; ### to this one

      $downloader->fetch("http://www.webaddress.com", \%siteHash); ## << This

      ## Will be a different hash to any similarly named hash above 
      ## unless it was shared.
   }
);

## Unless you are doing other stuff here, 
## this is just an exspensive synchronous call :)

$thread1->join;