our $re;
local $re = qr{
   \[
   (?:
      (?> [^\[\]]+ )
   |
      (??{ $re })
   )*
   \]
}x;

my $s = <<'__EOI__';
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
__EOI__

$s =~ s/$re/moo/g;

print($s);