MIME::Parser handles multiline subjects just fine.

#!/usr/bin/perl
use warnings;
use strict;

use Encode qw{ encode };
use LWP::Simple qw{ get };
use MIME::Parser;

my $parser = 'MIME::Parser'->new;
$parser->extract_uuencode(1);
$parser->extract_nested_messages(1);
$parser->output_to_core(1);

my $email = encode('UTF-8', get('https://pastebin.com/raw/LRs9J4pd'));
open my $f, '<', \$email or die $!;
my $e = $parser->parse($f);

print $e->head->get('Subject');

__END__
Output:
[updates-announce] MGASA-2023-0355: New chromium-browser-stable
 120.0.6099.129 fixes bugs and vulnerabilities
[download]

The problem is the code then processes the input file line by line. That's where the multiline subject is not processed correctly, I'd guess.

Update: It's hard to tell what exactly is the problem, as you didn't specify what error you got, and I'm not able to run your script without changes, as the Advisory module doesn't exist on CPAN.