Thank you your reply. The article is very detail. I know Buffering will give me the feeling of the dalaying, but the magical thing is: I called time() or Time::HiRes method before pipe call to capture the timestamp before the pipe opened, then I capture another timestamp after the pipe closed; while the program itself will display a time of execution as well. I found the time shown in perl is always less than the time shown in the pipe result largely......I checked the documentation and I am quite sure the time unit is correct and the method I used to get the interval is from the sample code, so I don't think this is the buffering problem...any idea?