If all started up, you will see that first column is very small!

First, when you distribute custom modules for testing, you should keep them in the working directory for easy cleanup and prevention of garbage collecting in PERL5LIB. So you could have just created a directory in your script directory, called Tk, put your custom module in it, and put " use lib '.' " in your test script.

Having done that, when I run your test script, I get

"$full_path" requires explicit package name......
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When I set an inital value to $full_path, like "my $full_path=0", it starts up and the first column, is very small.

Now that brings us to what is probably causing it. I'm just guessing here, but you are probably getting the default width for MListbox, not what you are passing in as the population hash. Why? Read "perldoc Tk::Derived" and pay close attention to the Populate sub

 sub Populate {
       my( $self, $args ) = @_;

       my $flag = delete $args->{-flag};
          if( defined $flag ) {
           # handle -flag => xxx which can only be done at create
           # time the delete above ensures that new() does not try
           # and do  $self->configure( -flag => xxx );
               }
......
.......
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It seems to me that your -width arg is getting lost somewhere in the way you populate your custom widget. Try Tk::Derived