The Specio distribution provides classes for representing type
constraints and coercion, along with syntax sugar for declaring
them.

Note that this is not a proper type system for Perl. Nothing in
this distribution will magically make the Perl interpreter start
checking a value's type on assignment to a variable. In fact,
there's no built-in way to apply a type to a variable at all.

Instead, you can explicitly check a value against a type, and
optionally coerce values to that type.