What things do you have to change/do to make code portable?

Perl already does most of the work for you. While glancing quickly over your code I noticed nothing platform specific, so chances are that you don't have to do anything at all.

Have I got the right idea for OO code?

Not entirely, I fear. You don't seem to use method calls at all. Consider $num_opt=getopt_dev->add_option($self,$opt,$type); - normally in OO you'd write $num_opt = $self->add_option($opt, $type); instead. That way one can actually subclass your module and override methods in the the derived module.

Frankly I'm not too fond on going through all of your code to check for other OO paradigms. If you provide a Synopsis (ie a small usage summary, as found in the documentation of all good CPAN modules) it will be much easier to judge if your approach is a good one.

A note on style: using an underscore in module names is generally frowned upon, use CamelCase instead, or separate the words with in the module name with :: instead.