You're focusing on all_attr() which probably isn't going to be of much use to you at this stage.
You probably want something like:
use strict;
use warnings;
use HTML::TreeBuilder;
my $data = do { local $/; <DATA> };
my $tree = HTML::TreeBuilder->new_from_content($data);
my $pre_tag = $tree->look_down("_tag", "pre");
print $pre_tag->as_text(), "\n";
Take a look at the the docs for Tree::Scanning for a basic tutorial and then have a look at the docs for HTML::Element since it is used alot by HTML::Treebuilder. HTML::Element has alot of methods that will be useful to you.
