Despite a lot of anecdote to the contrary, "PERL" was never really an acronym -- it's a "backronym". The name Perl was chosen first, then some people jokingly applied expansions to it, which caught on.
The PerlMonks community (highly recommended!) taught me the convention, and it's similar to Java's:
- It's never PERL (or JAVA)
- When you're talking about the language, it's Perl (or Java)
- When you're talking about the interpreter itself, it's perl (or java).
That said, it doesn't make a whole hill of beans if you do it "wrong".