Perl/Tk inspires many questions, many of them hard. This is a very basic one that you could find out on your own if you had access to a good reference or web page. This is something you learn very early with playing with pack, and i s mentioned in most every reference.

I hate to constantly Hawk "Mastering Perl Tk" by O'Reilly, but this is an excellent guide that will serve you well if you can pick it up. It gets my five out of five stars, and is really a great in-depth guide.

There should also be a wealth of web-based info out there, but I really can't comment on quality. I like my dead tree books. I would recommend we at PerlMonks but a Perl/Tk resources node on the Categorized Q&A page if there isn't such a node already. Sort of a "Read That Fine Manual", if you will.