Programming Perl, 4th Edition

Symbol Tables

The contents of a package are collectively called a symbol table. Symbol tables are stored in a hash whose name is the same as the package, but with two colons appended. The main symbol table’s name is thus %main::. Since main also happens to be the default package, Perl provides %:: as an abbreviation for %main::.

Likewise, the symbol table for the Red::Blue package is named %Red::Blue::. As it happens, the main symbol table contains all other top-level symbol tables, including itself, so %Red::Blue:: is also %main::Red::Blue::.

When we say that a symbol table “contains” another symbol table, we mean that it contains a reference to the other symbol table. Since main is the top-level package, it contains a reference to itself, making %main:: the same as %main::main::, and %main::main::main::, and so on, ad infinitum. It’s important to check for this special case if you write code that traverses all symbol tables.

Inside a symbol table’s hash, each key/value pair matches a variable name to its value. The keys are the symbol identifiers, and the values are the corresponding typeglobs. So when you use the *NAME typeglob notation, you’re really just accessing a value in the hash that holds the current package’s symbol table. In fact, the following have (nearly) the same effect:

*sym = *main::variable;
*sym = $main::{"variable"};

The first is more efficient because the main symbol table is accessed at compile time. It will also create a new typeglob by that name if none previously ...