I could be wrong, but it seems to be related to unicore/To/Upper.pl (at least that's what's being read immediately before — but you never know for sure, of course...), which might hint at some problem with processing certain unicode data.

In other words, if you could get a handle on what specific data Perl is processing at that moment, you might ideally be able to reproduce the problem outside of the RT/apache context, which in turn would likely ease further debugging...