It is very likely that you are simply failing to use Fcntl, which is where your constants (O_CREAT, etc.) are defined. If you don't already have it, put

use Fcntl;
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Update: Here's a demo showing that this is possibly the problem:

500,0 sauoq@fozzie:~$ perl -e 'sysopen(OH, "./foo", O_CREAT) or die $!
+'
No such file or directory at -e line 1.
501,2 sauoq@fozzie:~$ perl -MFcntl -e 'sysopen(OH, "./foo", O_CREAT) o
+r die $!'
502,0 sauoq@fozzie:~$
[download]

-sauoq
"My two cents aren't worth a dime.";