There was actually some talk of a CL backend for Perl 6, especially given the CLOS-like direction the metamodel has taken. However, CL's type declarations are mostly just hints for the compiler, and (as the spec says) the behavior is largely undefined when a value that does not match the type is assigned to a variable with an attached type declaration. In effect (from what I can deduce) this means there is no actual type "checking" in CL.

But regardless of this, since everything in Perl 6 will be an object, we basically thought of building it on top of CLOS. But alas, LISP has long suffered from several competing incomplete implementations and we could not find one with good enough (and fast enough) CLOS support to do this.