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

   package AbstractFoo;
    ...
    sub new {
...
    ...
    my $foo1 = new SubclassOfAbstractFoo(); # GOOD
    my $foo2 = new AbstractFoo();           # BAD

    package AbstractFoo;
    ...
    sub new {
...
           if $package eq "AbstractFoo";
        bless { @_ }, $package;
    }