If you're asking specifically about the default filehandles, they're all full caps,

If you are talking about STDIN, STDOUT and STDERR then perl has always (AFAIK) supported the lower case versions stdin, stdout and stderr although ARGV, ARGVOUT and DATA have to be upper case.

$ perl -le'print for grep /^std/i, %::'
stderr
stdout
stdin
STDOUT
STDERR
STDIN

$ perl -v

This is perl, v5.10.1 (*) built for i486-linux-gnu-thread-multi

Copyright 1987-2009, Larry Wall
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