Mastering Perl, 2nd Edition

Chapter 7. Symbol Tables and Typeglobs

Although I don’t normally deal with typeglobs or the symbol table, I need to understand them for use in later chapters. I’ll lay the foundation for advanced topics including dynamic subroutines and jury-rigging code in this chapter. Symbol tables organize and store Perl’s package (global) variables, and I can affect the symbol table through typeglobs. By messing with Perl’s variable bookkeeping I can do some powerful things. You’re probably already getting the benefit of some of these tricks without even knowing it.

Package and Lexical Variables

Before I get too far, I want to review the differences between package and lexical variables. The symbol table tracks package variables but not lexical variables. When I fiddle with the symbol table or typeglobs, I’m dealing with package variables. Package variables are also known as global variables since they are visible everywhere in the program.

In Learning Perl and Intermediate Perl, we used lexical variables whenever possible. We declared lexical variables with my, and those variables could only be seen inside their scope. Since lexical variables have limited reach, I didn’t need to know all of the program to avoid a variable name collision. Lexical variables are a bit faster too, since Perl doesn’t have to deal with the extra bookkeeping of the symbol table.

Lexical variables have a limited scope, and they affect only the part of the program where I define them. This little snippet declares the variable ...