Perl Module Basics

Now code that uses BOA::Logger can dispense with the package name:

use BOA::Logger;

open_log("logs/boa.log");

log_level(10);

write_log(5, "Hello log reader.");

write_log(15, "Debugging data here..");

Of course, the full package specification would still work you can always refer to 

BOA::Logger::write_log().

Now, BOA is a big application. In fact, BOA stands for big ol' application, so 

many other modules will be using BOA::Logger. Most of these modules will only be 

calling write_log(). The only code that will call open_log() and log_level() is the 

startup code. Fortunately users of the module can choose what symbols they want 

exported by providing a list of symbols to use:

use BOA::Logger qw(write_log);

Without this addition, a use BOA::Logger will import all exported symbols. To 

import nothing from a module that exports symbols by default, use an empty list:

use BOA::Logger ();

Subroutines aren't the only thing you can export. Variables can also be exported. 

For example, BOA::Logger could omit the log_level() subroutine and just export 

$LOG_LEVEL directly:

@EXPORT = qw(open_log $LOG_LEVEL write_log);

Now code that wants to set the logging level can import the $LOG_LEVEL variable 

and manipulate it directly:

use BOA::Logger qw($LOG_LEVEL write_log);

$LOG_LEVEL = 10;

write_log(10, "Log level set to 10.");

I'll return to the Exporter to provide more details in the next chapter.

BEGIN

Another problem with the BOA::Logger module is that other modules have to wait 

for open_log() to get called before they can use write_log(). This makes it difficult 

for modules to log their compilation and initialization. To solve this problem, the 

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