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

##</code><code>##

use base qw(AnInterface AnotherInterface YetAnotherInterface);

sub foo {
	my $self = shift;
	... code ...
};

##</code><code>##

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