Everybody said split, but note that if you have some punctuiation, you may be disappointed; for this sentence, for example, $words[2] is "split,".

You must have heard of CPAN: it has lots of modules. Lingua::EN::Splitter is one that (if you're looking for a real-world solution) can help you here. Of course, split *is* useful to know! But when it comes to human language, problems are often more complex than you think.