use strict;
use warnings;
use re 'eval';

my $begin = qr!\Q[xyz]!;
my $end   = qr!\Q[/xyz]!;

my @match;

my $re;
$re = qr{
	$begin
    (
        (?:
            (?> (?:(?!=$begin|$end).)+ )
            |
            (??{ $re })
        )*
	)
    $end
}x;

$_ = '[xyz][xyz]level 1.1 [xyz]level 2.1[/xyz] rest of 1.1 [/xyz]';

1 while s!$re!<xyz>$1</xyz>!;

print;

__END__
<xyz><xyz>level 1.1 [xyz]level 2.1</xyz> rest of 1.1 </xyz>