I'm using HTTP Watch through IE (I'm somewhat limited in what I can use in my corporate enviornment) to capture the data from the form submission -- I assume that HTTP Watch will capture the same data as LiveHttpHeaders. Here is the post data as captured by HTTP Watch:

paramater       value           size
login_password    passoword    18    
login_username    username    18    
prefLanguage    00000000    21    
submit        Log On            13
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Name    Direction Value Path    Domain          Duration   
C0    Sent      apc    /    .192.168.0.2    (Session)

HTTP Only    Secure 
No         Yes
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Header Sent                                      Value
(Request-Line)    POST /Forms/login1 HTTP/1.1
Accept    application/x-ms-application, image/jpeg, application/xaml+x
+ml, image/gif, image/pjpeg, application/x-ms-xbap, application/vnd.ms
+-excel, application/vnd.ms-powerpoint, application/msword, */*
Accept-Encoding    gzip, deflate
Accept-Language    en-US
Cache-Control    no-cache
Connection    Keep-Alive
Content-Length    73
Content-Type    application/x-www-form-urlencoded
Cookie    C0=apc
Host    192.168.0.2
Referer    https://192.168.0.2/logon.htm
User-Agent    Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64
+; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CL
+R 3.0.30729; .NET4.0C; MS-RTC LM 8; InfoPath.2)
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And then what? How are the two different?

Try this:

$mech->submit_form(

    .....
    fields => {
    .....
    submit => "Log On",
    },
),
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using these 3 variations:

    $data=$mech->submit_form(
        form_name => 'HashForm1',
        fields      => {
            login_password => 'password',
            login_username => 'username',
            prefLanguage => 00000000,
            submit => 'Log On',
        }
    );
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and this

    $mech->submit_form(
        form_name => 'HashForm1',
        fields => {
            login_password => 'password',
            login_username => 'username',
            prefLanguage => 00000000,

        },
        button => "submit",
    );
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and this

$mech->form_name('HashForm1');
$mech->field("login_username",'username');
$mech->field("login_password",'password');
$mech->field("prefLanguage",00000000);
$mech->submit_form();
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all yield the same 500 error. Trying to record the tranactions using httprecorder with the follwing line yields a 403

$mech->proxy(['https'], 'http://localhost:8080');
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but it does yield this in the log:

$agent->get('https://192.168.0.2'); $agent->get('https://192.168.0.2/home.htm'); $agent->get('https://192.168.0.2/logon.htm');

I have tried all of the different combinations of submitting the post data as well, even though that seemed like the least likely way to resolve the problem. Is it possible something in the headers isn't being set that would prevent log in? This seems like such a simple task and yet its turning out to be quite difficult.