sub foo {
    my $self = shift;
    croak "I am an instance method" unless
        $self and ref $self and $self->isa(__PACKAGE__);    
    # actual body of code
}

##</code><code>##

method sub {}              # $SELF=shift, check $SELF & isa
instance method sub {}     # =method modifier + ref $SELF
class method sub {}        # =method modifier + !ref $SELF & /\W/ & defined namespace