but the problem am facing is that @ each run of the loop it just scans the whole file and not the single line

I'm not entirely sure what you mean here. You mean grep (the command line tool) goes through the whole file every time? Uh, yeah, that's what it does. If you don't want that, you could rewrite grep in perl, I mean incorporate the functionality of grep that you want as a routine in your script, so that it works on only one line. It's not that hard, after all, ack is an extended grep which is written in Perl. Basically you just need to test the regexp on the line.