You can use callback (I hope I haven't twisted them into somthing else) that will do what you need. I use them in OOP and assign a default sub via a method, that is I create a method to return the default sub routine (action/code) if one isn't passed into the mehtod that uses the sub.
Here is the method that returns the default code (sub): 
sub default_code {
    my ($self,$extra) = @_;
    my $sub = sub { 
        my ($extra,$optional,$optional) = @_;
        # do something
        };
    return $sub;
}

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The method that might use this sub allows for a callback or overriding sub that is passed in via the method options. 
$object->display( { my_code => sub { 
                        my ($hashref) = shift;
                        foreach (keys %$hashref) {
                            # more code 
                        }
                             }; 
                   }
                 );

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Then in the display method 
sub display {
   my ($self,$args) = @_;
   # $args->{'my_code'} has our "custom code"
   # here is where we determine which one to use
   my $action = $args->{'my_code'} || $self->default_code;
   $action->($args);
}

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I don't know if this good style or not, but it has been working well for what I needed.

In reply to Re: Allowing the over riding of a program's design by trs80
in thread Allowing the over riding of a program's design by skazat

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