Sounds like there may be a problem with this: My script will do some add/edit/delete operation in each common.txt and make one new file called common_temp.txt under /tmp/ location. Maybe there is another program updating this common.txt file at the same time that you are reading it?

If you have one single Perl program running, it will just, like any other program language do one thing after another. Your terminology confuses me. What do you mean by: by the statement: or it just fires the statement n move to other statement?

I don't understand this part about "fires the statement n". Please use different words to describe this.