Chapter 2

Notice how the package variable $LOG_LEVEL is used to maintain state between 

calls to log_level() and write_log(). By state I mean that the module contains 

variables that store the value of past operations between calls to the interface. 

Thus the state of the module changes over time as the interface is used.

Here's a possible usage of the module, which would go in a separate script file:

# use the module

use BOA::Logger;

# open the log file

BOA::Logger::open_log("logs/boa.log");

# set the log level higher

BOA::Logger::log_level(10);

# write a log entry at level 5

BOA::Logger::write_log(5, "Hello log reader.");

# write a log entry at level 15   this won t be printed to the log

BOA::Logger::write_log(15, "Debugging data here.");

Exporting

BOA::Logger is useful enough, but it could be improved. For one thing, the module 

takes too much typing. To solve this problem, you can use the Exporter. The 

Exporter enables you to export symbols from your module into the package of the 

calling module. Exporting makes an entry in the calling package's symbol table 

that points to the called package. To export the three subroutines in BOA::Logger, 

you would change the top of the module source file, BOA/Logger.pm, to read as 

follows:

package BOA::Logger;

require Exporter;

@ISA = qw(Exporter);

@EXPORT = qw(open_log log_level write_log);

The second line loads the Exporter module require is commonly used here, but 

use also works. The third line accesses Perl's inheritance mechanism I'll describe 

inheritance in more detail in the  Object Oriented Modules  section, but for now 

you can just treat it as magic code that makes the Exporter work. Finally, the 

@EXPORT array is initialized with the list of symbols to export.

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