In general:

80% should be easy enough to achieve, use some rough regex to strip all variables with sigils and function calls and comments and count the built in commands...

A quick search didn't show any cpan modules for that (which is most likely due to my search strategy)

I know about JS libraries for syntax highlighting guessing the code.

The recommended technique is to train a classifier based on a "term frequency algo" (see tf-idf) with right and wrong code,( problem here is I don't know your wrong code, PHP should pose the biggest problem)

We also had a similar discussion in the past in order to decide if a poster forgot code tags.

update

After finding it again, I realized that it's so detailed that it merits an extra reply.