Perl is particularly well suited for gathering and manipulating text. This is why it is a favorite with system administrators as well as HTML authors. The following examples only begin to show how perl can be useful to a programmer.
Of course, perl is used extensively in serving up content when run in concert with a webserver. The webserver knows that certain URL's are meant to run perl scripts, and when it finds those URL's it runs the perl scripts for you automatically. The perl program creates output in the form of a webpage that in turn gets sent to the browser. The user sees a page created on the fly just for that request.
Perl can go and visit a website for you, without you ever using a browser. It can pull in the whole webpage, and hunt through it for bits of information. This info can then be repackaged into whatever form you need it: a different web-page, a printed report, or a piece of email.
The converse of this is that it can submit forms for you and interact with webservers at the other end. An example of this is an e-commerce transaction server; perl can send specific information to a payment server and then await the result to see whether the payment went through properly or not.
Okay, we lost our cat last year. The kids were really upset, and we were all distraught. Then I realized I could get perl to help me find the cat. I was able to write a perl script that went out over the Internet and collected and printed the name, address and phone number of all the houses on the streets in my neighborhood. With that list, we were then able to call every house one after the other. (We did eventually find the cat.) This is a concrete instance of the example above - visiting web sites, scraping info from them and assembling a report.
Perl can be used to construct web pages according to your own defined rules. As a matter of fact, the navigation links of this website were built with just such a tool. All the menus down the left hand side on the entire site are created by a perl script, and they can be updated to a completely different layout in a matter of minutes.
Perl scripts can make global changes to web pages. With the right script, presto-chango! wherever a particular phrase is found, it gets replaced with improved wording. Very broad yet precise changes are possible.
Perl comes with the capability to interact with online "backend" databases -- the massive storehouses of information running behind the scenes on large websites. It can search for database records containing particular pieces of information and it can add new records or update existing records. With the ability to interact with a user through a web browser, perl can act as the interface between the user and the database running on the server.
To start getting perl to do the things you want, find out how to run perl on your computer in the next section.