think i have to use tie::file.I went thru the perl doc ..but its not clear how to use it .could you give me a simple eg how to use it

for Tie::File, you have look at the file's lines as if they would be like "array elements", - by changing them you'd change the file. What did you try so far - what didn't work?

BTW, this problem looks rather simple, so you could employ your own file handling - sth. like this:

 ...
 my ($fnNew, $fnOld) = ('test.cfg', 'test.cfg.bak');
 rename $fnNew, $fnOld or die "can't rename $fnNew, $!";

 open my $fin,  '<', $fnOld || die "$fnOld - $!";
 open my $fout, '>', $fnNew || die "$fnNew - $!";
 
 while( my $line = <$fin> ) {
    $line =~ s/(newyork\s+)1536000/${1}7878787878/;
    print $fout $line
 }
 close $fin; close $fout;
 ...
[download]

This would do basically the same as the 'one-liner' in another example already given.

Regards

mwa