Quoting the Perl article on Wikipedia.
The name is normally capitalized (Perl) when referring to the language and uncapitalized (perl) when referring to the interpreter program itself since Unix-like file systems are case-sensitive. Before the release of the first edition of Programming Perl, it was common to refer to the language as perl; Randal L. Schwartz, however, capitalised the language's name in the book to make it stand out better when typeset. The case distinction was subsequently adopted by the community.
Also check the perlfaq about this question.
The correct casing is "Perl" for the language and "perl" for the executable. Using "PERL" flags you as someone who isn't particularly familiar with the language or community.
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".
While, as has been said, it doesn't make THAT much difference if you get it wrong, some folks do use correct capitalization (or at least, NOT referring to 'PERL' or any of the more sensible backcronyms) as a shibboleth for clue in job ads. :)
"PERL is a programming language for writing CGI applications. Its main strength is that it doesn't have any unnecessary warnings or strictures. It is a direct descendent of Perl, a programming language which was used mainly by programmers. However, the original language required too much reading and thinking and so PERL was developed as a language which was more in tune with the requirements of the Internet age".
Here's the answer from perlfaq1:
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." You may or may not choose to follow this usage. For example, parallelism means "awk and perl" and "Python and Perl" look OK, 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.
It's Perl (for the language) or perl
(for the interpreter) but NEVER 'PERL'!