Thank you POJ for the suggestion and showing how to do in an effective way :). Yes, I will use your code, the way you have done it is excillent. I was trying to do in another way, though I am not feeling proud to say my code is a good one. But just tried to make it work. But without your help it was not possible. Thanks to both POJ and PRYRT, your suggestion works for me and atleast I tried to think other options. Thanks again :).

my $pin_name;
my $out_pin;
my $outdir_pin;
my $flag_pin=0;
my $csvoutpin;
my $max_tran_val;

open (FILE2,"<","File_2") || die "Can not open Final_output.csv File";
my @csvFile = <FILE2>;

open (FILE1,"<","File_1") || die "Can not open libray File";
my @libFile = <FILE1>;


    for (my $i = 0; $i< @libFile; $i++)
    {
      my $line = $libFile[$i];
      chomp $line;
      if ($line =~m/^\s*pin\s*\(\"(.*)\"\)/g || $line4 =~m/^\s*pin\((.
+*)\)/g)
      {
       $pin_name = $1;
       chomp $pin_name;
      }
      if ($line =~m/^\s*direction\s*:\s*output/g)
      {
       $out_pin = $pin_name;
       $outdir_pin = $line;
       $flag_pin =1;
      }
      if  ($flag_pin == 1)
      {
       foreach my $csvoutpinLine (@csvFile) {
           chomp $csvoutpinLine;
           my @tempLine = split(',',$csvoutpinLine);
           my $csvoutpin=$tempLine[0];
           my $max_tran_val=$tempLine[1];
           if ($csvoutpin eq $out_pin) {
                $line =~ s/$outdir_pin/$outdir_pin\n      max_transiti
+on :$max_tran_val /g ;
                $libFile[$i] = $line;
            }
        }
       $flag_pin=0;
      }
    }
    
open (FILE3,">","File_3") || die "Can not open libray File";

    print FILE3 "@libFile" ;
    
close FILE1;
close FILE2;
close FILE3;
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