my url = "https://mysite.com";
my $mech = WWW::Mechanize->new();
$mech->get($url);
$mech->follow_link(text => "Welcome", n => 1);

my $f = HTML::Form->parse(<<EOT, $url);
<form action="signin.jsp" onsubmit="returnblah();" method="post" id="somethn_else">
<input type="text" name="username">user</input>
<input type="text" name="password">pass</input>
<input type="text" name="etc">etc</input>
</form>
EOT

# here is a workaround because of some javascript
$f->put_input("text", { name => "username" });
$f->value("username" => $username);
$f->put_input("text", { name => "password" });
$f->value("password" => $password);

$mech->submit();