A structured type constraint is a standard container Moose type constraint, such as an ArrayRef or HashRef, which has been enhanced to allow you to explicitly name all the allowed type constraints inside the structure. The generalized form is: TypeConstraint[@TypeParameters or %TypeParameters] Where 'TypeParameters' is an array reference or hash references of Moose::Meta::TypeConstraint objects. This type library enables structured type constraints. It is built on top of the MooseX::Types library system, so you should review the documentation for that if you are not familiar with it. WWW: http://search.cpan.org/dist/MooseX-Types-Structured/