Programming Perl, 4th Edition

Changing the Package

The notion of “current package” is both a compile-time and runtime concept. Most variable name lookups happen at compile time, but runtime lookups happen when symbolic references are dereferenced, and also when new bits of code are parsed under eval. In particular, when you eval a string, Perl knows which package the eval was invoked in and propagates that package inward when evaluating the string. (You can always switch to a different package inside the eval string, of course, since an eval string counts as a block, just like a file loaded in with do, require, or use.)

For this reason, every package declaration must declare a complete package name. No package name ever assumes any kind of implied “prefix”, even if (seemingly) it is declared within the scope of some other package declaration.

Alternatively, if an eval wants to find out what package it’s in, the special symbol _ _PACKAGE_ _ contains the current package name. Since you can treat it as a string, you could use it in a symbolic reference to access a package variable. But if you were doing that, chances are you should have declared the variable with our instead so it could be accessed as if it were a lexical.

Any variable not declared with my (or state) is associated with a package—even seemingly omnipresent variables like $_ and %SIG. Other variables use the current package, unless they are qualified:

$name = 'Amelia';           # name in current package

$Animal::name = 'Camelia';  # name in Animal package

The ...