Fellow Monks,
thanks for taking your time. I thought I had pointed out that I do not need help with a perl-code problem here. The answer I was hoping for and that I finally found myself was just, that this messages comes from the ISAPI module.

I do not know what may have caused it but I know that I do not have to deal with any perl internals (and the error message suggested something in that direction) nor that the code is a total mess.