- isa/can validation in XS allows this: { a => { isa => \%hash } } and tries to see if a->isa("HASH(0x1581)"). Instead, we should die if given a non-array reference. Same for can, etc. See how this is handled in callbacks for example. - make it possible to explicitly not use the XS version in a non-hacky way. - is there something better than "N/A" that could be used when validation is being done outside a sub? - add an excludes param to the spec as the opposite of depends, suggested by Diab Jerius (see rt.cpan.org).