use strict; # <- always start with this
use warnings; # <- this is good too
use WWW::Mechanize;

my $mech = WWW::Mechanize->new();
$mech->credentials( 'admin', 'password' );
$mech->get( 'http://192.168.1.1/' );
$mech->follow_link( test_regex => qr/configuration/ );
$mech->submit_form(
    form_number => 1,
    fields => {
        ip_address => '202.36.7.8',
    },
);