perl, all-lower case, refers to the perl executable. Examples:

• "I installed perl today",
• "I have version 5.8.8 of perl",
• "perl crashed again (not!)".

Perl, first letter capitalized, refers to the Perl language. Sometimes it's as broad as the entire Perl community. Examples:

• "Perl has no built-in switch statement, but it does have Switch.pm" (As of Perl 5.10, Perl does have a built-in switch statement: see Why you should upgrade to Perl 5.10),
• "You don't have to put parens around subroutine arguments in Perl",
• "I hate Perl (not!)".

Despite being a backronym, it is never correct to write PERL in all caps. Why? Honestly, because it's UGLY to scatter random all-caps NOUNS in your writing. It also hearkens back to things-which-are-written-in-all-caps-which-we-would-rather-forget, like COBOL or FORTRAN. Writing PERL in all caps is a strong red flag that the author does not know much Perl, or lives outside the Perl community who would have beaten charismatically deprogrammed such behavior out long ago.

It's worth noting that the "perl" vs "Perl" distinction is not strongly held by most and you're unlikely to raise an eyebrow by mixing them up. But saying "PERL" will raise ire.

contributed by Michael Schwern on Dec 12 3:21pm