#!/opt/perl/bin/perl

use strict;
use warnings;

use lib '.';
use MyModule;

# ----------
open(my $logfile, '>>', 'blah.log')
  or die "Couldn't open log file.";

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

# ----------
# Set up MyModule to use our logging function.
MyModule::set_logger_fn(\&my_logging_fn);

# ----------
# main
my_logging_fn("Doing something in the script.");
MyModule::do_some_work();
my_logging_fn("Ok, that's all for now.");

##</code><code>##

package MyModule;

# Before calling any functions in this module,
# be sure to first call `set_logger_fn(...)`.

my $log_fn_ref;

sub set_logger_fn {
    my ($fn_ref) = @_;
    $log_fn_ref = $fn_ref;
}

# ----------
sub do_some_work {
    # ...
    # write a log message.
    $log_fn_ref->("Doing some work in MyModule::do_some_work()");
    # ...
}

1;