sub InterfaceA::validate
{
  my ($candidate) = @_;
  error unless ref($candidate);
  error unless $candidate->isa("InterfaceA"); # explicit ISA
}

sub InterfaceB::validate
{
  my ($candidate) = @_;
  error unless ref($candidate);
  error unless $candidate->can("fn1"); # implicit ISA
  error unless $candidate->can("fn2");
}