use POSIX;
sub timenow_is_between
{
    my ( $bgn, $end ) = @_;  # args are "HH:MM" strings (length==5)
                             #  e.g. 20:30 or 07:00
    my $timenow = strftime( "%H:%M", localtime );
    if ( $bgn ge $end ) {
        return ( $timenow ge $bgn or $timenow lt $end );
    } else {
        return ( $timenow ge $bgn and $timenow lt $end );
    }
}

##</code><code>##

if ( timenow_is_between( "20:30", "07:00" ) {
    print "this program is running at night\n";
} else {
    print "it is not night time right now\n";
}