It operates on every line. On each line, there will be, at most, one substitution. If you want multiple substitutions on a line, add the 'g' modifier (see perlre for details) like this:

s/the line to be replaced/this is the new line/g
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Thanks Ken for your below link to tie and your answers. The reason I created my program in the above way Is so it only replaces the first line in the loop not every line it loop . It seems crazy but I need this in the program I am creating.

If it's only for the first line, use this instead:

use strict;
use warnings;

use Tie::File;

tie my @xfil, 'Tie::File', 'var.txt' or die $!;

$xfil[0] =~ s/the line to be replaced/this is the new line/;

untie @xfil;
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Substantially less cumber. :-)

-- Ken

Hi Ken, Your example of tie was awesome . I figured out how to fix my code with your example. I did not use tie But you really helped me and I sure do appreciate it

Thanks Ken for your below link to tie and your answers. The reason I created my program in the above way Is so it only replaces the first line in the loop not every line it loop . It seems crazy but I need this in the program I am creating.

Thanks ken for the code . I really appreciate it.