use re 'eval';

$begin = '[xyz]'; $end = '[/xyz]';
$string = '[xyz]level 1.1 [xyz]level 2.1[/xyz] rest of 1.1 [/xyz]';
$re = qr{
    \Q$begin\E
        (?:
            (?> (?:(?!=\Q$begin\E|\Q$end\E).)+ )
            |
            (??{ $re })
        )*
    \Q$end\E
}x;

print 'Yeah!' if $string =~ $re;