The book "Mastering Perl/Tk" is the best learning guide out there. Once you plow through that, you can search groups.google.com for snippets that have been posted before. Search for example: "Perl/Tk Text limit lines"
There is a tagline for one of the monks here, that goes something like: "99% of the code needed has already been written"...... you just need to search google and groups.google to find it.
I always do a groups.google.com search whenever I have a question about code or syntax..... I almost always find the answer. You will see the code I posted( original by Marc Dashevsky, in comp.lang.perl.tk )