More definitive references on CPAN Dev releases welcome.

Probably worth mentioning that CPAN::Meta::Spec now includes the release_status required field. The linked doc includes the proscription:

If the version field contains an underscore character, then release_status must not be "stable."

and finishes the section with:

Consumers may use this field to determine how to index the distribution for CPAN or other repositories in addition to or in replacement of heuristics based on version number or file name.

I see this as a belt-and-braces facility with the added bonus of being able to specify "unstable" as a value to indicate really alpha releases.