our $qr=qr/<(?:[<>\\]+|\\.|(??{$qr}))*>/;
print "not " unless '<<><><<<>>><>>'=~/^($qr)$/;
print "ok - $1\n";

##</code><code>##

print "not " unless '<<><><<<>>><>>'=~/^((?&:<(?:[<>\\]+|\\.|(?:&))*>))$/;
print "ok - $1\n";

##</code><code>##

my $rules=qr/(?&& # compile this stuff, but dont match it
             (&foo: .... ) # define ...
             (&bar: .... ) #  ... some rules
             )
           /x;
if ($blah=~/(&foo)(&bar)$rules/) { ... }