It should be noted that expansion of 'perl' as an acronym was invented long after the language was named. I've never been entirely clear what the difference is between 'perl' and 'Perl', but I'm pretty sure there is no such thing as 'PERL'.

From perlfaq1:

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 isn't really an acronym, apocryphal folklore and post-facto expansions notwithstanding.

I beg to differ on the "long after the language was named" part, unless Larry named Perl long before releasing it in the wild. The manual page released with the very first version of Perl, version 1.0.0, says:

PERL(1)                                                   PERL(1)



NAME
       perl - Practical Extraction and Report Language
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Abigail

It should be noted that expansion of 'perl' as an acronym was invented long after the language was named.

True enough, but its understandable that people think its the acronym (instead of a "backronym" or "retronym") when the first thing they see in "perldoc perl" is:

PERL(1)             2002-06-10 (perl v5.6.1)              PERL(1)


NAME
     perl - Practical Extraction and Report Language
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HoA is correct, because the o does not start a "significant" word. (Your English teacher will tell you that a word is "significant" for capitalisation purposes unless it is an article, a coordinating conjunction, or a short preposition. I forget whether "short" in this context means <5 letters or <=5 letters.) It is common for abbreviations to capitalise only the words that start a significant word: e.g., ComIntern, CoBOL[1].

We don't write PERL, of course, because computer technical words are (usually) case-sensitive. This is a special rule, but that's normal in English. Nouns in the field of music follow a different declension if they end in "o"; words and abbreviations imported unchanged from Latin, unlike words imported from all other languages, are typeset in italics, except for mathematical terms imported from Latin (e.g., QED), which are not; et cetera, ad infinitum.

The case-sensitivity in computer words allows for a significant amount of disambiguation: hence the distinction between Perl and perl, the lack of any confusion between BIND and activities with rope, and so on. The rule is universal among people who understand computer stuff in general, which is an ever-growing percentage of the population, and I am confident that it will become a permanent special rule like the others listed. But yes, people who don't know will continue to write "PERL", "Email", "pianoes", "ect.", confuse e.g. with i.e., and just generally write like wankers.

1. CoBOL does not follow hackish rules because (like RPG) it is not a hacker language.

for(unpack("C*",'GGGG?GGGG?O__\?WccW?{GCw?Wcc{?Wcc~?Wcc{?~cc'
.'W?')){$j=$_-63;++$a;for$p(0..7){$h[$p][$a]=$j%2;$j/=2}}for$
p(0..7){for$a(1..45){$_=($h[$p-1][$a])?'#':' ';print}print$/}
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