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;