I get an error.
In your original post you said you can not login and in this one you say you get an error. Keep in mind that we can't see what you see so in order to help we need to know what is not working, how it is not working, and what error messages you get. In this case, I believe I made a typo which you could have fixed by RTFMing. Here is the corrected code that should work.
my $url = 'http://192.168.0.1/'; # Change if necessary
my $mech = WWW::Mechanize->new( autocheck => 1 );
'admin' => 'secret'
$mech->get( $url );
# Rest of code goes here
had an almost identical problem which you can read about here