You used the debugger? Excellent. What did you find out? I think one very interesting question would be: How does the function know it is called the last time? Is it because you are at the last "page" of your excel file and there is some closing data missing? Or is it because some final cleanup damages your data? What happens if you read that file into Excel, maybe append some data and save it again ?

Can you rule out (through your debugging) the possibility that you feed the wrong data to the excel module on your last call of the function? If yes, the bug is not in the code you reproduced

You could also change the parameters of your graphs to see if there is some condition necessary to activate the bug. What happens if you don't do the last insert_chart?

PS: Did it occur to you that you didn't even tell us which CPAN module you use to create the graphs. Maybe there is only one and everybody knows it, but I wouldn't count on that.