You can't use grep for that, but the following should be a good enough template for a start:

#!/usr/bin/perl -w
use strict;

while (<>) {
  if (/foo/) {
    # ... change line :
    $_ = "This line was changed ($_)";
  };
};
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Perl also has the facility to edit files in place, which is quite convenient for oneliners. The above could be written as one line like this:

perl -pe '/foo/ and $_ = "This line was changed ($_)"'
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I hope that helps you, as I could not really make much out of your question. A module which is really good for manipulating files line by line would be Tie::File - take a look at the documentation, it is worth a read!

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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