use HTTP::Cookies; $agent->cookie_jar(HTTP::Cookies->new);
[download]

WWW::Mechanize creates a cookie_jar like that by default.

It hasn't always done that, and there could be a problem with it. Doing it explicitly helps debug.

May be this is ignore_discard?

use HTTP::Cookies;

my $cookie_jar = HTTP::Cookies->new( ignore_discard => 1 );
$mech->cookie_jar($cookie_jar);
[download]

That would only affect saving cookies to a file, like after the object is destroyed (like when a program ends, between sessions).

This work for me:

#!/usr/bin/perl

use strict;
use warnings;
use WWW::Mechanize;

my $mech = WWW::Mechanize->new();

$mech->get("https://utdirect.utexas.edu/registrar/nrclav/results.WBX")
+;

$mech->field( "PASSWORDS", "pass", 2 );

$mech->submit_form(
    
    form_name => "logonform",
    fields => {
        
        LOGON => "rc32536",
    },
);

$mech->get("https://utdirect.utexas.edu/registrar/nrclav/results.WBX")
+;
[download]

Second get needed because of JavaScript redirect to this URL on the response page.

#!/usr/bin/perl --
use strict;
use warnings;

use WWW::Mechanize;

my $ua = WWW::Mechanize->new;
$ua->get(qw! https://utdirect.utexas.edu/registrar/nrclav !);

print "\n\n",
$ua->response->status_line, " ", $ua->uri, "\n\n",
$ua->cookie_jar->as_string, "\n\n";

$ua->set_visible(qw' TESTusername TESTpassword');

print $ua->current_form->dump,"\n\n";

$ua->submit;

print "\n\n", $ua->response->request->as_string, "\n\n",
$ua->cookie_jar->as_string, "\n\n";

print "Error: $1 " if $ua->content =~ m{\Q<span class="error">\E(.+?)\
+Q</span>\E}s;


__END__

200 OK https://utdirect.utexas.edu/registrar/nrclav/index.WBX

Set-Cookie3: DOC="/registrar/nrclav/index.WBX"; path="/"; domain=.utex
+as.edu; path_spec; discard; version=0
Set-Cookie3: FC=NONE; path="/"; domain=.utexas.edu; path_spec; discard
+; version=0
Set-Cookie3: PDLK=NONE; path="/"; domain=.utexas.edu; path_spec; disca
+rd; version=0
Set-Cookie3: SC=NONE; path="/"; domain=.utexas.edu; path_spec; discard
+; version=0


POST https://utdirect.utexas.edu/security-443/logon_check.logonform [l
+ogonform]
  CDT=20090331212639             (hidden readonly)
  NEW_PASSWORD=                  (hidden readonly)
  CONFIRM_NEW_PASSWORD=          (hidden readonly)
  PASSWORDS=                     (hidden readonly)
  LOGON=TESTusername             (text)
  PASSWORDS=TESTpassword         (password)
  <NONAME>=Log In                (submit)




POST https://utdirect.utexas.edu/security-443/logon_check.logonform
Accept-Encoding: gzip
Referer: https://utdirect.utexas.edu/registrar/nrclav
User-Agent: WWW-Mechanize/1.54
Content-Length: 107
Content-Type: application/x-www-form-urlencoded
Cookie: SC=NONE; PDLK=NONE; FC=NONE; DOC=/registrar/nrclav/index.WBX
Cookie2: $Version="1"

CDT=20090331212639&NEW_PASSWORD=&CONFIRM_NEW_PASSWORD=&PASSWORDS=&LOGO
+N=TESTusername&PASSWORDS=TESTpassword


Set-Cookie3: DOC="/registrar/nrclav/index.WBX"; path="/"; domain=.utex
+as.edu; path_spec; discard; version=0
Set-Cookie3: FC=NONE; path="/"; domain=.utexas.edu; path_spec; discard
+; version=0
Set-Cookie3: PDLK=NONE; path="/"; domain=.utexas.edu; path_spec; disca
+rd; version=0
Set-Cookie3: SC=NONE; path="/"; domain=.utexas.edu; path_spec; discard
+; version=0
Set-Cookie3: TS=20090331212639; path="/"; domain=.utexas.edu; path_spe
+c; discard; version=0


Error: 

          An error has occurred. You are not at fault. This error has 
+automatically been reported to the System Administrator. Please try a
+gain later.
[download]