I don't want to include new module to do this stuff. Reason is I am going to add this stuff in 1000 lines of code which does other operation.

:) Like they say, do what makes sense to you

But you might be interested in these other viewpoints :)

You could always use open/readline ... see Re: modifying a file with regex!

Or the instant classics on the subject of wheels :) Yes, even you can use CPAN Top 11 (GOOD) reasons not to use someone else's Modules, Top Seven (Bad) Reasons Not To Use Modules