use Fcntl qw(:DEFAULT :flock);

sysopen(FH, $file_banner, O_RDWR|O_CREAT) or die "Can't open $file_banner: $!\n";
flock(FH, LOCK_EX)                     or die "Can't write-lock $file_banner: $!\n";
$num = <FH> || 0;                      # do not use 'or' here
seek(FH, 0, 0)                         or die "Can't rewind $file_banner: $!\n";
truncate(FH,0)                         or die "Can't truncate $file_banner: $!\n";
print FH $num+1, "\n"                  or die "Can't write to $file_banner: $!\n";
close(FH)                              or die "Can't close $file_banner: $!\n";