I have already reported this bug before for an older version of apache and mod_perl, but the problem is still there. The CGI version is that of perl-5.005. My Perl CGI script occasionally produces the above error (Undefined subroutine CGI::thead). Restarting or reloading Apache will fix the problem for some time. It also seems that modifying unrelated parts of the Perl script affects the probability of the occurrence significantly. So I guess there's a larger bug in the symbol table management of mod_perl. The script runs perfect when run without mod_perl. In addition the problem seems to happen on some Apache work processes (spawned servers), while others work fine. That is if I have four server processes running and the bug appears, I can just call the same routine again and again, until a different server processes it, then it will work. Reloading seems to cure all servers for a while. Unfortunately the script is non-trivial, and it requires additional environment setup. Regards, Ulrich P.S: I'm using the binaries from SuSE Linux 7.0 Professional with all patches installed. SuSE refused to provide a more recent Perl.
