they may help you clean up the code enough to let you see where the real problem lies.

Dear BrowserUk,
I don't know how to thank you! I have learnt so much from your latest reply. Just realized I'm still not as good as I thought I am in coding Perl --- by far.

Also I really appreciate your offer to look over my overall code (sp_neg_eff.pm) and the dataset.

I wouldn't dare to cause you more trouble after such kindness. I guess by cleaning up my code above I can figure out what's the problem.