No good reason. Purely muscle memory that I picked up decades ago. Maybe I saw someone else do it[1] and just picked up a bad habit. I'll stop the all caps and write Perl from now on.
Because they don't know much about contemporary Perl. It's actually a really good indicator of if someone actually knows anything about the language post the 90s. I used it as a soft hiring filter on CVs. You knew almost as matter of fact any dev putting that on their CV was self taught from monster nests of legacy.
[1] http://www.perlmonks.org/index.pl?node_id=511722