Good choice! You might want to take a look at how (not) to ask a question. It will help you get better answers and advice here.

Anyway, I recommend the following books:

Learning Perl and Programming Perl

Also, perl has some excellent documention. As for file downloads, I'm not sure what you mean exactly, but the CGI.pm documentation is probably useful.

edit (a day later): Zaxo and mulander posted some very good comments. Take a good look at their comments.