Clue 1: If you google for the error message or call "perldoc perldiag" on the command line you will find out that the error message means that your second argument to bless (your $package above) is a reference instead of a string with a class name in it. Check where the error occurs (line xxx) and retrace the program back to where $package gets its value. My guess is that you forgot to change something when you copied the code to create the 2nd derived class
Clue 2: in your URL class you bless $url to your class but never initialize it with anything. Maybe you had $up in mind (which gets initialized). Something similar seems to be happening in your derived class as well
Clue 3: Generous use of Data::Dumper to print out your data structures will give you insight in what you constructed. Check your variables whether they contain what you expect them to contain.
Clue 4: "perldoc perlobj" has lots of examples of the optimal use of object initialization (lines 40 to 90 for example). As you can see you have it almost right, only some bugs need to be corrected it seems
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