#!/usr/bin/perl -w

use strict;
use WWW::Mechanize::Firefox;
my $mech = WWW::Mechanize::Firefox->new(
    autoclose => 0,
    autodie => 1,
);
$mech->get('https://www-304.ibm.com/pkmslogin.form');
sleep 7;
my $dummy; #found that with only username and password #parameters in set_visible function it entered only #password in username field so added this dummy variable.

my $username = 'someusername';
my $password = 'somepassword';

$mech->set_visible( $dummy, $username, $password );

$mech->click('Submit');

#tried below statements as well

$mech->click('submit');

$mech->click({ xpath => '//button[@name="Submit"]' });