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

##</code><code>##

sub foo # requires interfaces InterfaceA, InterfaceB
{
  my ($self) = @_;
  InterfaceA::validate($self);
  InterfaceB::validate($self);
}

##</code><code>##

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");
}

##</code><code>##

sub validate_object
{
  my ($self, @interfaces) = @_;
  error unless ref($self);
  foreach my $interface (@interfaces)
  {
    my $fn = $interface . "::is_implemnted_by";
    error unless $fn->($self);
  }
}

#...

sub foo
{
  my ($self) = @_;
  validate_object($self, qw/InterfaceA InterfaceB InterfaceC/);
}

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