It would be best to simply call tmpnam() once

No, the point is that tmpnam() can give you the name of a file that doesn't exist and then, by the time you get a chance to call sysopen, someone else has created a file with that name. This is called a race condition.

Specifying O_EXCL to sysopen means that it will fail if the race condition happens so that you can retry, which means requesting a new file name.

But I wouldn't use the original code because, for one thing, it will just loop forever if sysopen is failing for a reason other than "file already exists". So I'd go with File::Temp (which isn't included with Perl [update: prior to version 5.6.1], unfortunately). Despite there being lots of methods avaiable for creating temporary files in Perl, it appears that only File::Temp "gets things right" and gets rid of the race conditions in a portable manner. Though I haven't investigated this thoroughly so I'd be happy to be proved wrong on that. (:

        - tye (but my friends call me "Tye")