My guess is that you wouldn't find it in the perl module source because the heavy lifting is done in the expat code - which is c code.

taking a very cursory look at my install, I see a file:

/usr/local/src/XML-Parser-2.31/expat-1.95.4/xmlwf/xmlfile.c

which contains lines:

#ifdef _DEBUG
#define READ_SIZE 16
#else
#define READ_SIZE (1024*8)
#endif
[download]

I'm not certain that this is the issue, especialy since the size given seems smaller than what you've found, but this might be where you need to start.

--Bob Niederman, http://bob-n.com