package Base;

sub new {
    # Creates a new instance of the class
}

sub attrs {
    # When called as a class method, will keep track of attribute names
}

sub make_attr_subs {
    # Called by attrs() to create the mutators
}

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package Singleton;

sub new {
    # Instead of creating a new instance, will return an already created one.
}

--------------------

package ReadOnly;

sub make_attr_subs {
    # Instead of creating just a mutator, will create an accessor X() and a mutator _X().
}