Advanced Perl Programming

Chapter 3. Typeglobs and Symbol Tables

We are symbols, and inhabit symbols.

—Ralph Waldo Emerson

This chapter discusses typeglobs, the symbol table, filehandles, formats, and the differences between dynamic and lexical scoping. At first sight, these topics may seem to lack a common theme, but as it happens, they are intimately tied to typeglobs and symbol tables.

Typeglobs are immensely useful. They allow us to efficiently create aliases of symbols, which is the basis for a very important module called Exporter that is used in a large number of freely available modules. Typeglobs can also be aliased to ordinary references in such a way that you don’t have to use the dereferencing syntax; this is not only easier on the eye, it is faster too. At the same time, using typeglobs without understanding how they work can lead to a particularly painful problem called variable suicide. This might explain why most Perl literature gives typeglobs very little attention.

Closely related to typeglobs and symbol tables is the subject of dynamic versus lexical scoping (using local versus my). There are a couple of useful idioms that arise from these differences.

This is the only chapter that starts off by giving a picture of what is going on inside, rather than first presenting examples that you can use directly. The hope is that you will find the subsequent discussions really easy to follow.

Perl Variables, Symbol Table, and Scoping

Variables are either global or lexical (those tagged with my). ...