To enter input into any input field, focus the input field by ->click()ing it, and then send keys.

Also, I assume that you can switch input fields by using the \t tab key.

I try but not work, it still doesn't detect and the button is blocked, in addition as I display the content after filling it is empty

        $mech->click({xpath => '//input[@name="username"]', single => 
+1 });
        $mech->sendkeys( string => $user );
        $mech->click({xpath => '//input[@name="password"]', single => 
+1 });
        $mech->sendkeys( string => $pass );

        $mech->sleep(1);

        $mech->click({intrapage => 1, text => 'Log In', single => 1 })
+;
        $mech->sleep(2);

        print $mech->content;
[download]

Without seeing the HTML and Javascript, it's hard to know what the page does wrong so that clicking in the form fields get filled but the button does not get unblocked.

If by "the content is empty" you mean that the input you type is not visible in the form fields, then either that does not work or you are not clicking on the correct fields.

You can indirectly watch your browser as it progresses by using the screencast feature. See Mojolicious::Plugin::PNGCast for something that will display a window of your headless browser page.