Use Data::Dumper with expensive data in perl

$logger->debug(sub { "large data structure is: ".Dumper( expensive_calculation($foo) ) });

This ensures that expensive_calculation() is not performed unless the log level is DEBUG or below.

Pattern specification for Log4perl log messages

Pattern specification for Log4perl log messages:
http://search.cpan.org/~mschilli/Log-Log4perl-1.32/lib/Log/Log4perl/Layout/PatternLayout.pm

Simple log4perl config

log4perl.rootLogger=DEBUG, STDERR
log4perl.appender.STDERR=Log::Log4perl::Appender::Screen
log4perl.appender.STDERR.layout=PatternLayout
log4perl.appender.STDERR.layout.ConversionPattern=[%d] [%p %c] - %m%n

Using an XML configuration file for Log::Log4perl

XML configuration file:

<!--
* General configuration for log4perl
* LOGFILE.filename may be set by the application
* All logging goes to STDOUT (CONSOLE)
-->
<log4perl>
<log4perl.rootlogger>WARN, CONSOLE, LOGFILE</log4perl.rootLogger>

<log4perl.appender.console>Log::Log4perl::Appender::Screen</log4perl.appender.CONSOLE>
<log4perl.appender.console.layout>PatternLayout</log4perl.appender.CONSOLE.layout>
<log4perl.appender.console.layout.conversionpattern>[%d] [%p %c] - %m%n</log4perl.appender.CONSOLE.layout.ConversionPattern>

<log4perl.appender.logfile>Log::Log4perl::Appender::File</log4perl.appender.LOGFILE>
<!-- Let logging come to STDERR on the console, and *pipe* into a log so that unexpected errors are caught
<log4perl.appender.logfile.filename>/optional/absolute/path/to/logfile.log</log4perl.appender.LOGFILE.filename>
-->
<log4perl.appender.logfile.mode>append</log4perl.appender.LOGFILE.mode>
<log4perl.appender.logfile.layout>PatternLayout</log4perl.appender.LOGFILE.layout>
<log4perl.appender.logfile.layout.conversionpattern>[%d] [%p %c] - %m%n</log4perl.appender.LOGFILE.layout.ConversionPattern>

<!-- Add different log levels for specified modules
<log4perl.logger.clickthrough.transform>INFO</log4perl.logger.ClickThrough.Transform>
-->
</log4perl>

How to read the configuration:

my $log4perl = XML::Simple->new(ForceArray=>0, KeyAttr=>[])->XMLin( $log4perl_config_filename );
my $logname = $0;
$logname =~ s/\.pl$/.log/;
$log4perl->{'log4perl.appender.LOGFILE.filename'} = $logname;
Log::Log4perl::init($log4perl);
my $logger = Log::Log4perl->get_logger(__PACKAGE__);

Different files for log4perl levels

log4perl.rootLogger=DEBUG,CONSOLE,ViewableLog,DebugLog,ErrorLog

###################################################################################
# console - what to watch
log4perl.appender.CONSOLE=Log::Log4perl::Appender::Screen
log4perl.appender.CONSOLE.layout=PatternLayout
log4perl.appender.CONSOLE.layout.ConversionPattern=[%d] [%p %c] - %m%n
log4perl.appender.CONSOLE.Filter = ViewableFilter

  log4perl.filter.ViewableFilter        = sub {    \
       my %p = @_;                           \
       $p{log4p_level} eq "FATAL" or          \
       $p{log4p_level} eq "ERROR" or          \
       $p{log4p_level} eq "INFO" or          \
       $p{log4p_level} eq "WARN"           \
                                          }

###################################################################################
# Viewable log - record of CONSOLE
log4perl.appender.ViewableLog=Log::Log4perl::Appender::File
log4perl.appender.ViewableLog.layout=PatternLayout
log4perl.appender.ViewableLog.layout.ConversionPattern=[%d] [%p %c] - %m%n
log4perl.appender.ViewableLog.filename = logs/output.log
log4perl.appender.ViewableLog.Filter = ViewableFilter

###################################################################################
# Error log - warnings and errors
log4perl.appender.ErrorLog=Log::Log4perl::Appender::File
log4perl.appender.ErrorLog.layout=PatternLayout
log4perl.appender.ErrorLog.layout.ConversionPattern=[%d] [%p %c] - %m%n
log4perl.appender.ErrorLog.filename = logs/error.log
log4perl.appender.ErrorLog.Filter = ErrorFilter

  log4perl.filter.ErrorFilter        = sub {    \
       my %p = @_;                           \
       $p{log4p_level} eq "FATAL" or          \
       $p{log4p_level} eq "ERROR" or          \
       $p{log4p_level} eq "WARN"           \
                                          }

###################################################################################
# Debug Log - everything
 log4perl.appender.DebugLog = Log::Log4perl::Appender::File
 log4perl.appender.DebugLog.layout=PatternLayout
 log4perl.appender.DebugLog.layout.ConversionPattern=[%d] [%p %c] - %m%n
  log4perl.appender.DebugLog.filename = logs/debug.log
  log4perl.appender.DebugLog.Filter = AllFilter

  log4perl.filter.AllFilter        = sub {    \
       my %p = @_;                           \
       $p{log4p_level} eq "FATAL" or          \
       $p{log4p_level} eq "ERROR" or          \
       $p{log4p_level} eq "WARN" or          \
       $p{log4p_level} eq "INFO" or          \
       $p{log4p_level} eq "DEBUG" or          \
       $p{log4p_level} eq "TRACE"           \
                                          }

###################################################################################
# To enable DEBUG-level logging for a particular module (and any modules that subclass it),
# un-comment the following and change e.g. 'TempTopicsAdminTools.ModuleManager' to the your own module.

##log4perl.logger.TempTopicsAdminTools.ModuleManager=DEBUG
#log4perl.logger.TempTopicsAdminTools.PageManager=WARN
#log4perl.logger.TempTopicsAdminTools.CategoryManager=WARN