You are advised to use LWP. Here is a sample. It works, but I just threw in the authorization lines, you may need to tweak them.

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

my $url ='http://zentara.zentara.net/~zentara/cgi-bin/uploadz';
my $file = 'testout.tgz';

my $ua = new LWP::UserAgent;

my $req = POST $url,
Content_Type => 'multipart/form-data',
Content => [
file => [$file]
];

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


my $res = $ua->request($req);
if ($res->is_success){print $res->as_string;
}else{print $res->status_line;
}
exit 0;
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