<h1>Software error:</h1>
<pre>Can't call method &quot;click&quot; on an undefined value at C:/P
+erl/site/lib/WWW/Mechanize.pm line 982, &lt;STDIN&gt; line 1.
</pre>
<p>
For help, please send mail to this site's webmaster, giving this error
+ message 
and the time and date of the error.
</p>
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What is the output of:

perl -MWWW::Mechanize -e 'print $WWW::Mechanize::VERSION ."\n";'
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What is the URL you're hitting.

https://hungergj.home.xs4all.nl/test.htm

This is the perl program "test button.pl":

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

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

$ua->get("https:\/\/hungergj.home.xs4all.nl\/test.htm"); 
$ua->click_button( number => 1 ); 
print $ua->title;
print "=================\n";;
$_=<STDIN>;
[download]

"click_button: No form has been selected at test button.pl line 12."
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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]

---

🦛

You didn't answer the other question. If sharing the URL isn't suitable you'll need to do some basic debugging.