Do you realize you're running a debug build of Perl? That makes it slower. On the plus side, the checks that it slower is what found the problem and tells us exactly where the problem occurred.

Specifically, on a blank line. Line 1741 of Perl 5.10.0's sv.c is a blank line.

You don't appear to running stock Perl. Without having the sources that were used to build your Perl, all we can tell is that some function is supposed to be passed a scalar that contains a string (i.e. a scalar with SVf_POK set), but it was passed something different.

This is either a bug in Perl, or a bug in an XS module. Try upgrading to Perl 5.10.1. Lots of bugs were fixed in 5.10.1.

However, there's a good chance it's related to the unresolved bug you mentioned. The file handle fields from CGI are very special. They act as a string or a file handle depending on how you use them. The ticket talks of globs, and globs are closely related to file handles. Can you somehow avoid including this field in the session?

Update: Added last paragraph.