Programming Perl, 4th Edition

Avant-Garde Compiler, Retro Interpreter

There’s a right time to think about everything; sometimes that time is beforehand, and sometimes it’s after. Sometimes it’s somewhere in the middle. Perl doesn’t presume to know when it’s the right time to think, so it gives the programmer a number of options for telling it when to think. Other times it knows that some sort of thinking is necessary but doesn’t have any idea what it ought to think, so it needs ways of asking your program. Your program answers these kinds of questions by defining subroutines with names appropriate to what Perl is trying to find out.

Not only can the compiler call into the interpreter when it wants to be forward thinking, but the interpreter can also call back to the compiler when it wants to revise history. Your program can use several operators to call back into the compiler. Like the compiler, the interpreter can also call into named subroutines when it wants to find things out. Because of all this give and take between the compiler, the interpreter, and your program, you need to be aware of what things happen when. First we’ll talk about when these named subroutines are triggered.

In Chapter 10 we saw how a package’s AUTOLOAD subroutine is triggered when an undefined function in that package is called. In Chapter 12 we met the DESTROY method, which is invoked when an object’s memory is about to be automatically reclaimed by Perl. And in Chapter 14 we encountered the many functions implicitly called when a tied ...