Reverted to YAML::Syck and it worked. Tried to boil the problem with YAML::XS down. Worked through all the indata and when attaching a particular part of the data to a working YAML file it stopped working. Thing is if I try to just use that part or any other it works, so the data can't be wrong. I can't see anything special with that part (its about 2MB, each block is working seperatly , could be it since the other part I use were smaller). With the same data but generated with Syck it works as a charm. Syck seems to ignore CompressSeries flag so I bet that could be the problem. Or at least my data were generated without it. Thanks again for pointing me to this. Ill never use Dumper again ;)