The error message you got shows that you have attempted to run the molwt.pl which needs the Bio::DB::GenBank Module, it doesn't show if BioPerl is installed or if some libraries got misplaced inadvertently, so show what you have attempted and what other errors encountered for these might be more relevant to your question.

Installing BioPerl can be thorny at times since not all the packages needed for the installation are available from a single repository such as the CPAN, in fact, related libraries can be distributed over different repositories, so you got to connect to these repositories and maybe manage them on and off too, these quick tips can lead you to primers on these areas, try it out and it will work out..