my %escapes = (
   '&lab;' => '<',
   '&rab;' => '>',
   '&lcb;' => '(',
   '&rcb;' => ')',
   '&lsb;' => '[',
   '&rsb;' => ']',
);

sub final_processing {
   my ($content) = @_;
   $content =~ s{
      (?: (&[lr][acs]b;)
      |   <special...>(...)</special>
      |   <post_include...>(...)</post_include>
      )
   }{
      if (defined($1)) {
         $escapes{$1}
      }
      elsif (defined($2)) {
         '&'.$2.';'
      }
      else {
         ...
      }
   }xeg;

   return $content;
}