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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You sure can, see this post for editing in place using grep and Tie::File.


cp
----
"Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic."

I don't mean to be rude but a good place to start would be the documentation. There are examples of how to use grep there. If you don't understand the docs then come and ask questions. Perlmonks is a place where you have to put some effort into finding the answer yourself, rather than copying the answer from someone else. Its better to learn something than to copy it :)

Give this a try:

#!/usr/bin/perl
use strict;
use warnings;
use Tie::File;

my @file;
tie @file, 'Tie::File', '/tmp/filename'
  or die "Failed to tie file: $!";

foreach my $matching_line ( grep { m/^myLine/ } @file ){
  $matching_line =~ s/^myLine/this_was_a_match/;
}
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use strict;

open(INFILE, "/path/to/file");
my @lines = map {s/oldthing/newthing/ if(m/myLine/);$_;} <INFILE>;
close(INFILE);

open(OUTFILE, ">/path/to/file");
print OUTFILE $_ foreach(@lines);
close(OUTFILE);
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I am using this, but does not do want I want:

if(/^otsort/)
{ 
   $myLine =~ s|(/apost.*) f\(fix,(\d{4}\).*$|"$1,record\($2\)"|g; 
   print "$myLine\n";  
   print outFile "$endUnixLine\n"; 
}
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