It may be a bit dated but the Perl Cookbook is a very good resource.

Here is a fairly up to date OO guide
http://perldoc.perl.org/perlootut.html

It is still good to know the old school way of Perl OO.
http://perldoc.perl.org/perlobj.html

Pretty much everything here
http://perldoc.perl.org/index-language.html
and here
http://perldoc.perl.org/index-tutorials.html
is the most up to date information on programming in Perl.

This may not be the internals of the C code that Perl is built with, but it is the internals of how Perl is built and how the syntax works together with the design of the Perl language.

Finally, don't go crazy trying to fit everything into a bloated OO design. Composition is often preferred over inheritance. Work on the basics before worring too much about the finer points. Perl to a large extent is about breaking things into modules and trying to use built-in functionality to manipulate data in chunks, rather than byte by byte as one would do in C or C++. Perl Programmers spend a lot of time figuring out how not to use for loops. And they have a particular love of hashes, regular expressions and the sort function.

The true "A" team Perl shops I've worked in, Modern Perl and Higher-Order Perl were the books that formed the basis of our design style. I rate Modern Perl the more important of the two.