I don't think that it's a closed standard either. Rather, like Perl 5, it seems to be a non-standardised, or, perhaps less pejoratively, implementation-defined language.

On the other hand, somewhat like Perl, it is also a language intended from its beginning to be extended. Knuth has said that he never expected things like LaTeX to be built on top of TeX; he rather thought that people would hack directly on the TeX source as necessary.