package MyLoggingModule;

use strict;
use warnings;

# ----------
my $logfile;

# ----------
sub init {
    my ($logfile_name) = @_;
    open($logfile, '>>', $logfile_name)
      or die "Couldn't open log file.";
}

sub loggit {
    my ($log_msg) = @_;
    print {$logfile} $log_msg, "\n";
}

# ----------
1;

##</code><code>##

#!/opt/perl/bin/perl

use strict;
use warnings;

use lib '.';
use MyLoggingModule;
use MyModule;
use MyOtherModule;

# ----------
MyLoggingModule::init('blah.log');
MyLoggingModule::loggit("Doing something in the script...");
MyModule::foo();
MyOtherModule::bar();
MyLoggingModule::loggit("My work here is done.");

##</code><code>##

package MyModule;

use strict;
use warnings;

use lib '.';
use MyLoggingModule;

# ----------
sub foo {
    # ...
    # write a log message.
    MyLoggingModule::loggit("Doing some work in MyModule::foo()");
    # ...
}

1;

##</code><code>##

package MyOtherModule;

use strict;
use warnings;

use lib '.';
use MyLoggingModule;

# ----------
sub bar {
    # ...
    # write a log message.
    MyLoggingModule::loggit("Doing some work in MyOtherModule::bar()");
    # ...
}

1;