hello again,
i tried the following things:
- adding $graph_shape->OLEFormat->Activate; after the call to addOleObject.
- adding a $Presentation->Close() and PptApp->Quit() at the end of the script.
- adding a $graph_shape->Application->Quit() after a graph insertion.
Alas it didn't change anything. i checked too if the slide have different names, theorically yes, i set the the $slide->{name} property according to an indice, but later i wasn't able to find the slide names i set in powerpoint. But I think it's ok with slide names.
Moreover I found that if i insert 2 or 3 slides, there's always one that is ok, i can edit the values normally.

So i investigate more, i'll post a comment if i found, but i 'm a bit pessimistic as i don't really know what is the root cause of this.