Hi samira_saber. First things first: Please edit your reply to add code tags around your code! Please see Markup in the Monastery and Writeup Formatting Tips; also see How do I change/delete my post? for general node-editing etiquette.

Some comments (in no particular order):

• ... it is not working ...

  This is almost useless as a problem description. What is "it"? Is it the code you have posted? How does it not work? Is there an error or warning message or messages? Under what conditions of input, etc., etc.? (Later in your reply you write "... before that i read the variable from the text file and print it and it worked." I don't understand this statement, especially in the context of your statement that "it" is not working.)
• The code in your reply does not have the lines
      use warnings;
      use strict;
  at the start of the source code. These provide very important support and protection for a novice Perler — and for elder Monks as well; I rarely write write a Perl program without using them. See warnings and strict. Use of these modules (called pragmas by convention) is referred to as "enabling warnings and strictures."
• Your code statement
      my $filename_in = "topol1.top";
  means you are hard-coding the input file name, whereas I was taking the file name from a command line argument. Indeed, the code still requires at least one command line argument (assumed to be the file name) to be present when the script is invoked; the statement
      die "usage: perl $0 file.name \n" unless @ARGV;
  enforces this (see perlvar for info on the  @ARGV array).

• ... i omitted the line for the $sol.

  In my original code, the my variable  $sol1 holds the string to be substituted in the  s/// substitution operation.
      s{ (SOL) [^\n*] }{$1 $sol1}xmsg;
  I don't understand the point of the substitution operation without something to substitute. Further, if strictures had been enabled (again, see strict), the code would not even compile without this variable being defined and in scope. Even without strictures enabled, warnings would have complained (a run-time warning, not a fatal error) if an undeclared variable having an undefined value had been used — if warnings had been enabled.

• ... it is not working ... I down know why ...

  This is always useful: Basic debugging checklist.

I hope these comments will be helpful. If you have any questions, please don't hesitate to ask.