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sub foo
{
  my ($self) = @_;
  croak "..." unless ref($self) && $self->isa("AnInterface")
}

sub foo # requires interfaces InterfaceA, InterfaceB
{
...
  InterfaceA::validate($self);
  InterfaceB::validate($self);
}

sub InterfaceA::validate
{
...
  error unless $candidate->can("fn1"); # implicit ISA
  error unless $candidate->can("fn2");
}

sub validate_object
{
...
  my ($self) = @_;
  validate_object($self, map {"Interface$_"} qw/A B D/);
}