I have a question regarding whether a file is currently opened or not (unix). I've written a perl utility which does a directory comparison and picks out any new files that have appeared since it's previous run. I need to check the contents of these new files but only if they are not currently still open by some other process. These files are being written by an Oracle application not by another Perl utility.
I tried using flock but this doesn't seem to work e.g.

while (defined($_ = readdir(AUDDIR))) {
    next if /^\.\.?$/;
    unless (delete $files{$_}) {
      open(FH, "<$_") or die "Can't open file $_: $!\n";
      flock(FH, LOCK_EX) or warn "Can't get exclusive lock on file $_ 
+: $!\n";
      close(FH);
    }
}
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Notice that in this code I am using exclusive lock mode (even though all I want to do is read the contents). If I use LOCK_SH it grants a shared lock even if the file is currently openned by another process. Using LOCK_EX I get an error

Can't get exclusive lock on file ora_6688.aud : Bad file number
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This happens regardless of whether the file is still openned or not.
As it happens in this case the filenames contain the unix pid of the process which wrote (or is still writing) to the file.
I could get a list of all the current pid's and compare it to the list of pid's in the new filenames to see if the filename in question is still open, but this seems a messy way of doing it to me.
Any suggestions on how I can handle this problem would be welcome