click_button says "Has the effect of clicking a button on the current form by specifying its attributes". The page at that URL doesn't have a form, so you get the expected error message.

Just use the links instead as previously explained:

#!/usr/bin/env perl 
use strict;
use warnings;
use WWW::Mechanize;

my $ua = WWW::Mechanize->new();

$ua->get ('https://hungergj.home.xs4all.nl/test.htm');
my @links = $ua->links();
print $ua->get ($links[0]->url)->decoded_content;
[download]