sub _fail(@)
{
    ### DEBUG
    if (open my $log, ">>", "/tmp/qmail-queue.log") {
        print $log "@_\n";  # or something like this
        close $log;
    } else {
        # writing another error message to nowhere if
        # this fails would be mostly useless...
    }
    ###

    my $default_ec = shift;
    ...
}