My first guess is that the first line works because of auto-vivification. In that line, you refer to $o{0} without trying to use the value as a hash reference itself. This triggers FETCH, but doesn't actually use the returned value anywhere. After this initial call to FETCH it appears to work (according to your testimony-- I don't have a MSWin32 box to try and reproduce this on).

In the second case, you are trying to directly utilize the value from $o{0} as a hash reference. I'm not sure what part of your FETCH routine is the source of the problem, but that would be the place to start looking.

--rjray