As myocom pointed out, '?node=somethinghere' is a CGI query using method GET. It doesn't necessarily come from an HTML form, anchors and links can be written with such queries in their href.

The CGI script's basic job is to examine the query parameters, decide what to do, and print the correct HTML response page. Each of those stages can depend on the name/value pairs passed to the script.

A few tips:

1. use warnings;
2. use strict;
3. use CGI;
4. Taint mode should be on.
5. Know what you expect to see in parameters, and ignore anything unexpected.

1, 2, and 4 are there to drive you nuts, but it's worth it. They go a long way toward assuring stable and correct behavior. 3 is a convenience which is superior to home-rolled ways of accessing CGI parameters. 5 works with taint mode to prevent clients from persuading your script to do unexpected things.

Good luck, have fun.

After Compline,
Zaxo