That's not true since v5.10 (2007). Here's a sample that matches a subset of LaTeX markup:

  $matcher = qr{
      (?&File)
   
      (?(DEFINE)
          (?<File>     (?&Element)* )
   
          (?<Element>  \s* (?&Command)
                    |  \s* (?&Literal)
          )
   
          (?<Command>  \\ \s* (?&Literal) \s* (?&Options)? \s* (?&Args)? )
   
          (?<Options>  \[ \s* (?:(?&Option) (?:\s*,\s* (?&Option) )*)? \s* \])
   
          (?<Args>     \{ \s* (?&Element)* \s* \}  )
   
          (?<Option>   \s* [^][\$&%#_{}~^\s,]+     )
   
          (?<Literal>  \s* [^][\$&%#_{}~^\s]+      )
      )
  }xms