#!/usr/bin/perl

use strict;
use warnings;

use Regexp::Common;

$_ = <<'--';
[xyz]level 1.1[/xyz]
[xyz]level 1.2[/xyz]
[xyz]level 1.1 [xyz]level 2.1[/xyz] rest of 1.1 [/xyz]
[xyz] error [xyz] level 1.1 [xyz] level 2.1 [/xyz] [/xyz]
[xyz] level 1.2 [/xyz]
--

1 while s!$RE{balanced}{-begin => "[xyz]"}{-end => "[/xyz]"}{-keep}!
          "<table><tr><td>" . substr ($1, 5, -6) . "</td></tr></table>"!gex;

print;
__END__
<table><tr><td>level 1.1</td></tr></table>
<table><tr><td>level 1.2</td></tr></table>
<table><tr><td>level 1.1 <table><tr><td>level 2.1</td></tr></table> rest of 1.1 </td></tr></table>
[xyz] error <table><tr><td> level 1.1 <table><tr><td> level 2.1 </td></tr></table> </td></tr></table>
<table><tr><td> level 1.2 </td></tr></table>