#!/usr/bin/perl
use strict;
use warnings;
use 5.010;
use Mojo::DOM;
my $dom = Mojo::DOM->new(do { local $/; <DATA> });
$dom->find('ul')->each(sub {
        $_->append_content('<span class="backTotop"> <a href="#a4">Back to top</a></span>');

});
say $dom->to_xml;

__DATA__
<ul>
    <li class="orangeTxt1"><a name="a3" id="a3">question-1</a></li>
    <li>Answer1</li>
</ul>
<ul>
    <li class="orangeTxt1"><a name="a4" id="a4">question-2</a></li>
    <li>Answer2</li>
</ul>