From language/nativecall

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sub nativecast($target-type, $source) is export(:DEFAULT)

This will cast the Pointer $source to an object of $target-type. The source pointer will typically have been obtained from a call to a native subroutine that returns a pointer or as a member of a struct, this may be specified as void * in the C library definition for instance, but you may also cast from a pointer to a less specific type to a more specific one.

As a special case, if a Signature is supplied as $target-type then a subroutine will be returned which will call the native function pointed to by $source in the same way as a subroutine declared with the native trait. This is described in Function Pointers.