I know this is on old thread; but this is probably the best place to post what I have found.

One should use "Perl" instead of "PERL" because, as a language, especially a technical language, the identification without ambiguity of the semantic structure is of high importance.

See the Object Modeling Group's "Common Warehouse Metamodel Specification" (OMG CWM v1.1 3/2003) Section 4.3.2.9: Core Metamodel: Expression.
"An Expression is a statement that will evaluate to a ... set of instances when executed in a context."

That context is identified by a named language;
"In general, a language should be spelled and capitalized exactly as it appears in the document defining the language. For example, use COBOL, not Cobol; use Ada, not ADA; use PostScript, not Postscript."

And I would add: use Perl, not PERL.