What's the difference between "perl" and "Perl"?

One bit. Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to signify the language proper and "perl" the implementation of it, i.e. the current interpreter. Hence Tom's quip that "Nothing but perl can parse Perl."

Before the first edition of Programming perl, people commonly referred to the language as "perl", and its name appeared that way in the title because it referred to the interpreter. In the book, Randal Schwartz capitalised the language's name to make it stand out better when typeset. This convention was adopted by the community, and the second edition became Programming Perl, using the capitalized version of the name to refer to the language.

You may or may not choose to follow this usage. For example, parallelism means "awk and perl" and "Python and Perl" look good, while "awk and Perl" and "Python and perl" do not. But never write "PERL", because perl is not an acronym, apocryphal folklore and post-facto expansions notwithstanding.