Input data do not seem to be a valid HTML.

<h3>
    <table><tr>BBBBB 01/08/2018
</h3>                                           # </H3> is nested insi
+de the table
        <td>236</td><td>002</td><td>121</td>
        <td>266</td></tr>
</table>
[download]

With that out of the way... This is how you could do it using Mojo::DOM.

use strict;
use warnings;
use Mojo::DOM;
use Data::Dumper;
use feature qw/say/;

my $input = join '', map { $_ } <DATA>;
my $dom = Mojo::DOM->new($input);
my @results;

for my $h3 ($dom->find('h3')->each) {    
    # skip everything but the heading we are after
    next unless $h3->all_text =~ /^\s* BBBBB \s+/ix;    
    
    # assume that immediately after the heading we have the table we a
+re interested in
    my $table = $h3->following->first;    
    $table->find('td')->each( sub { push @results, $_->all_text} );
}

say Dumper \@results;

__DATA__
<h3>AAA 28/07/2018</h3>
<table><tr><td>1351</td><td>990</td><td>783</td><td>523</td></tr></tab
+le>

<h3>BBBBB 01/08/2018</h3>
<table><tr><td>236</td><td>002</td><td>121</td><td>266</td></tr></tabl
+e>

<h3>KK K P 25/07/2018</h3>
<table><tr><td>200</td><td>3345</td><td>667</td><td>137</td></tr></tab
+le>
[download]