1 while s/<([^<]+?)>([^<]*?)<\/>/
   $_{$1}->{$2} ||= do {
      #TODO: Make sure $1 doesn't contains "::".
      my $func = $pkg . "::" . $1;
      #TODO: Make sure $func exists.
      no strict 'refs';
      $func->($2)
   }
/e;

##</code><code>##

my %cb = (
   use => \&use,
   dbl => \&dbl,
);

1 while s/<([^<]+?)>([^<]*?)<\/>/
   #TODO: Make sure $cb->{$1} exists.
   $_{$1}->{$2} ||= $cb->{$1}->($2)
/e;