Note that parsing (X|HT)ML with regexes is fragile; you'd do better to use an XML parser, about which others can advise better than I. However:

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 use Data::Dump qw(dd);
 ;;
 my $s = '<tr><td>99</td></tr><h3><table><tr>BBBBB 01/08/2018</h3>'
       . '<td>236</td><td>002</td><td>121</td><td>266</td></tr>'
       . '<tr><td>999</td><td>9999</td></tr>'
       ;
 print qq{[[$s]] \n};
 ;;
 my @n = $s =~ m{ (?: BBBBB [^>]+ > | \G) <td> (\d+) </td> }xmsg;
 dd \@n;
"
[[<tr><td>99</td></tr><h3><table><tr>BBBBB 01/08/2018</h3><td>236</td>
+<td>002</td><td>121</td><td>266</td></tr>
<tr><td>999</td><td>9999</td></tr>]]

[236, "002", 121, 266]
[download]

Update: A few afterthoughts:

• The  use 5.010; statement enforcing a minimum Perl version of 5.10.0 is not needed. I originally used a regex operator introduced with 5.10, but got rid of it, so you can lose the statement too.
• It might be wise to add more delimitation before and after the  BBBBB keying substring in the regex:
      m{ (?: <tr> BBBBB \b [^>]+ > | \G) <td> (\d+) </td> }xmsg
  This makes regex parsing of this HTML only slightly less problematic.
• Both these changes tested under Perl version 5.8.9.

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Give a man a fish:  <%-{-{-{-<

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