use WWW::Mechanize::Firefox;
my $mech = WWW::Mechanize::Firefox->new(
    launch => '/path/to/firefox',
);
$mech->get('https://login.binck.nl/klanten/Login.aspx?ReturnUrl=%2fklanten%2fdefault.aspx');

$mech->eval(q{document.getElementById('ctl00_Content_Gebruikersnaam').value='username';document.getElementById('ctl00_Content_Wachtwoord').value='password';
window.open(document.getElementById('ctl00_Content_LoginButton').href, '_self', 'true');});