Programming Perl, 4th Edition

Chapter 14. Tied Variables

Some human endeavors require a disguise. Sometimes the intent is to deceive, but more often the intent is to communicate something true at a deeper level. For instance, many job interviewers expect you to dress up in a tie to indicate that you’re seriously interested in fitting in, even though both of you know you’ll never wear a tie on the job. It’s odd when you think about it: tying a piece of cloth around your neck can magically get you a job. In Perl culture, the tie operator plays a similar role: it lets you create a seemingly normal variable that, behind the disguise, is actually a full-fledged Perl object that is expected to have an interesting personality of its own. It’s just an odd bit of magic, like pulling Bugs Bunny out of a hat.

Put another way, the funny characters $, @, %, or * in front of a variable name tell Perl and its programmers a great deal—they each imply a particular set of archetypal behaviors. You can warp those behaviors in various useful ways with tie, by associating the variable with a class that implements a new set of behaviors. For instance, you can create a regular Perl hash, and then tie it to a class that makes the hash into a database, so that when you read values from the hash, Perl magically fetches data from an external database file, and when you set values in the hash, Perl magically stores data in the external database file. In this case, “magically” means “transparently doing something very complicated”. You know ...