Sorry that this may not help too much but we have non-threaded Perl with Oracle::DBD and Oracle8 and Oracle9 clients with no issues like you are reporting. This is on Solaris SPARC v8.

This may well be something to do with Solaris though.

The problem we did have is using 64bit Oracle and libraries. Most code puts 32bit in lib and 64bit in lib64. Oracle puts 64bit in lib and 32bit in lib32. You just need to make sure that when building you get the right versions.