Mouse is Moose minus the antlers. (Moose is "A postmodern object system for Perl 5".) Moose is wonderful. Unfortunately, it's a little slow. Though significant progress has been made over the years, the compile time penalty is a non-starter for some applications. Mouse aims to alleviate this by providing a subset of Moose's functionality, faster. In particular, "has" in Moose is missing only a few expert-level features.