That url is not what you think it is, it goes to 404. Sadly that page is generated from the previous page at http://www.deviantart.com. Anyway get the action and method of the page, and the names of the login fields and send them in a POST method. You may consider using WWW::Mechanize as it makes this a little bit easier.

The form is a POST, and you should probably pass in the username, password, ref (set to http://www.deviantart.com), and reusetoken (set to 1) fields to the form which is https://www.deviartart.com/users/login/). You'll need to store the cookies from that transaction and pass them along with any requests you make to other pages.

use strict;
use warnings;

use LWP;
use HTTP::Request::Common qw(POST);

my $cookies = {};

my $ua = LWP::UserAgent->new( cookie_jar => $cookies,
          agent =>     'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 
+5.1');
my $req = POST 'https://www.deviantart.com/users/login',
                  [ username   => $mydauser,
                    password   => $mydapass,
                    reusetoken => 1,
                    ref        => 'http://www.deviantart.com'];
my $response = $ua->request($req);
print $response->content;
[download]

This is how it should work,, you may need to set a proper User Agent and such. I have also arranged for a cookie-jar to store your cookies. See LWP::UserAgent for more details.

Update: I changed the target of the POST to be https://www.deviantart.com/users/login from https://www.deviantart.com/users/login/ as login/ returned 404.