package AbstactFoo;
    ...
    sub new {
        die "Don't create any Abstract Foos direct, fool!";
    }

##</code><code>##

   package AbstractFoo;
    ...
    sub new {
        my $package = shift;
        bless { @_ }, $package;
    }
    ...
    my $foo1 = new SubclassOfAbstractFoo(); # GOOD
    my $foo2 = new AbstractFoo();           # BAD

##</code><code>##

    package AbstractFoo;
    ...
    sub new {
        my $package = shift;
        die "Don't subclass AbstractFoo directly!"
           if $package eq "AbstractFoo";
        bless { @_ }, $package;
    }