I'm a bit rusty on cgi fileuploads, but maybe you don't need WWW::Mechanize just to upload a file with basic_auth handled by the webserver. You seem to have a redundant attempt at auth. Try this:

#!/usr/bin/perl
use warnings;
use strict;
use LWP::UserAgent;
use HTTP::Cookies;
use HTTP::Request::Common qw(POST);

#test on plain http

my $https_post = 'https://zentara.zentara.net/~zentara/cgi-bin/uploads
+/up1.cgi';
my $https_user = 'zentara';
my $https_pass = 'foobar';
my $file = 'testout.tgz';


&postHTTPS();

sub postHTTPS
{
        my $ua = new LWP::UserAgent;
        $ua->protocols_allowed( [ 'http'] );
        $ua->cookie_jar(HTTP::Cookies->new(file =>".cookies.txt",autos
+ave => 1));

  #setup request
        my $req = POST($https_post, 
            Content_Type => 'multipart/form-data',
            
            Content =>[            
                   file =>[ $file  ],
                  ], );

         my $user = 'zentara';
         my $pass = 'foobar';
         $req->authorization_basic($user, $pass);

  #do post         
     my $response = $ua->request($req);
     if ($response->is_error())
            {
                    printf " %s\n", $response->status_line;
                    print "https request error!\n";
            } else {
                    my $content = $response->content();
                    print "$content\n";
                }

         if ( $response->is_success ) {
                   print $response->as_string;
         }else {
               print $response->status_line;
         }

}
[download]