...
<h1>404 Not Found</h1>
<h1>404 Not Found</h1>
<h1>404 Not Found</h1>
<h1>404 Not Found</h1>
<h1>404 Not Found</h1>
<h1>404 Not Found</h1>
Can't call method "as_HTML" on an undefined value at lwp.pl line 28, <$fh> line 184.

#!/usr/bin/perl

use strict;
use warnings;
use utf8;
use feature qw/say/;
use LWP::UserAgent;
use HTML::TreeBuilder 5 -weak;

my $ua = LWP::UserAgent->new;
$ua->agent("$0/0.1 " . $ua->agent);
$ua->agent("Mozilla/5.0");

open my $fh, '<', 'aa.txt' or die $!;
...