Because 'use signatures','use Function::Signatures','use Method::Signature::Simple' and 'use MooseX::Method::Signatures' all exist, and work well, but the CPAN

Because 'use signatures','use Function::Signatures','use Method::Signature::Simple' and 'use MooseX::Method::Signatures' all exist, and work well, but the CPAN cage match still continues for the One True Way. And since that effectively means that adding it into core would merely be an optimisation, it's preferred to wait until a standard has been settled upon so we don't end up having to maintain compatibility with something people have since discovered is a bad idea. The keyword parsing hooks added for version 12 make adding new keywords to experiment even easier, as well - so I think you can expect some signature form to go into core by v16 at the latest (2012), with the advantage that because the current solutions are CPAN modules your code using signatures of one of those sorts today won't break if/when that happens. edit: and as for chromatic's tweet, this is not "apologetics for lack of a feature", this is a personal explanation of why I haven't attempted to write a patch for this yet, but will be seriously considering doing so at some point unless somebody else gets there first. And as schwern points out, there's movement towards a unified syntax for invoking such things that we can quite possibly repurpose later.