Perl Module Basics

Functional Modules

The most obvious way to build a module is to place subroutines in the module and 

document them as the module's interface. For example, here's a module that 

provides a logging facility for a fictional application called BOA:

package BOA::Logger;

$LOG_LEVEL = 1; # default log level is 1

# open log file

sub open_log {

   my $filename = shift;

   open(LOG_FILE, ">>$filename") or die "Unable to open $filename : $!";

   print LOG_FILE "BOA log started: " . localtime(time) . "\n";

}

# set logging level

sub log_level { $LOG_LEVEL = shift; }

# write a log message if level is set high enough

sub write_log {

   my ($level, $message) = @_;

   print LOG_FILE "$message\n" if $level <= $LOG_LEVEL;

}

1;

CAUTION A real logging module would use flock() to prevent file 

corruption, but that would make these examples twice as long! The 

code in this chapter is kept as simple as possible real production code 

would need significant enhancement.

The concept for the module is simple BOA::Logger will provide logging at 

varying levels of detail known as log levels. The module's interface consists of three 

subroutines open_log(), log_level(), and write_log(). The application must call 

open_log() before the first call to write_log(). When a piece of code calls write_log(),

it provides two arguments, $level and $message itself. If $level is less than or equal 

to the currently set log level, the message is printed to the log. The log level defaults to 

1 and the application can change the value using the log_level() subroutine.

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