Mastering Perl, 2nd Edition

Chapter 16. The Magic of Tied Variables

Perl lets me hook into its variables through a mechanism it calls tying. Tied variables go back to the basics. I can decide what Perl will do when I store or fetch values from a variable. Behind the scenes, I have to implement the logic for all of the variable’s behavior. Since I can do that, I can make what look like normal variables do anything that I can program (and that’s quite a bit). Although I might use a lot of magic on the inside, tied variables have the familiar behavior that users expect. Not only that, tied variables work throughout the Perl API. Even Perl’s internal workings with the variable use the tied behavior.

They Look Like Normal Variables

You may have already have seen tied variables in action, even without using tie. The dbmopen command ties a hash to a database file:

dbmopen %DBHASH, "some_file", 0644;

That’s old-school Perl, though. Since then the numbers and types of these on-disk hashes has proliferated and improved. Each implementation solves some problem in another one, and they mostly live in CPAN modules now.

If I want to use an alternate implementation instead of the implementation Perl wants to use with dbmopen, I use tie to associate my hash with the right module:

tie %DBHASH, 'SDBM_File', $filename, $flags, $mode;

There’s some hidden magic here. The programmer sees the %DBHASH variable, which acts just like a normal hash. To make it work out, though, Perl maintains a “secret object” that it associates ...