use POSIX qw/strftime/;

# Main routine...
my $logfile = "program-%Y-%m-%d.log";
&do_log($logfile, "This is the message I want to log");

sub do_log
{
    my $logfile = shift;
    my $message = shift;

    open LOG, ">>".strftime($logfile, localtime);
    print LOG stftime("%H:%M:%S ", localtime);
    print LOG $message,"\n";
    close LOG;
}