main_module is bad for perl releases

[haarg]

Releases of the perl dist have an assortment of modules set as their main_module, such as less or utf8. These values are inconsistent, not useful, and misleading.

The most natural value for the field is probably perl, but that is not actually a module so it could be problematic.

Another option is leaving the field empty.

Whatever value is best, we'll probably need to have a special case for the perl dist. This seems fine to me.