PPI::Structure::Unknown is class for braces whose type is unknown, or
It primarily exists temporarily inside the lexer. Although some types of
braces can be determined immediately at opening, there are a number of
different brace types that can only be correctly identified after the
braces are closed.
A structure is typed as unknown during this period it is indeterminate.
A PPI::Structure::Unknown object should not <B>everB> make it out of the
lexer without being converted to its final type. Any time you encounter
this class in a PDOM tree it should be considered a bug and reported