Well, you could monkey-patch a tmpname implementation into the POSIX namespace.

BUT:

The POSIX module promises to provide you with an interface "to access all (or nearly all) the standard POSIX 1003.1 identifiers.". So everything in the POSIX namespace should / shall / must behave according to POSIX. Implementing a POSIX::tmpnam() that does something completely different will cause even more trouble.

POSIX has specified the behaviour of tmpnam(), probably after it was implemented everywhere. And unfortunately, tmpnam() just sucks. The problem is explained in https://linux.die.net/man/3/tmpnam and other places. Short: You build huge gapping security holes when using tmpnam(). Quoting the man page:

Bugs

Never use this function.

The clean way to handle temp files is File::Temp, and use the file handles instead of file names. Of course, this will require changes to the existing code. But those changes will remove the security holes introduced by tmpnam().

Alexander