You might like to take a look at Simon Cozen's PDF of his Perl 5 Internals course notes as mentioned here.

I enjoyed reading it very much. It is easy to get through, not too long, and covers topics like what yacc is, what basic functions are in what files, how the parse tree is set up, how regular expressions are computed, and lots of one-liner experiments you can do to see the structures created internally for variables and data structures.

You may also be interested in reading thoughts on Larry Wall quotes which cover some of the philosophical points mentioned in more detail in Larry Wall's Onion documents among others.