use warnings;
use strict;

die "usage: perl $0 file.name \n" unless @ARGV;

my $filename_in  = shift;
print "processing '$filename_in' \n";

my $filename_out = "$filename_in.new";

open my $fh_in,  '<', $filename_in  or die "opening '$filename_in': $!
+";
open my $fh_out, '>', $filename_out or die "opening '$filename_out': $
+!";

my $sol1 = 'my new stuff';

while (<$fh_in>) {
  s{ (SOL) [^\n]* }{$1 $sol1}xmsg;
  print $fh_out $_ or die "writing '$filename_out': $!";
  }

close $fh_in  or die "closing '$filename_in': $!";
close $fh_out or die "closing '$filename_out': $!";

rename $filename_in, "$filename_in.bak" or die "renaming '$filename_in
+': $!";
rename $filename_out, $filename_in      or die "renaming '$filename_ou
+t': $!";
[download]

c:\@Work\Perl\monks\samira_saber>dir
 Volume in drive C is Acer
 Volume Serial Number is 9480-355B

 Directory of c:\@Work\Perl\monks\samira_saber

08/14/2017  05:23 PM    <DIR>          .
08/14/2017  05:23 PM    <DIR>          ..
08/14/2017  05:04 PM               759 process_file_1.pl
08/14/2017  04:40 PM                46 topol1.au
08/14/2017  04:40 PM                46 topol1.top
               3 File(s)            851 bytes
               2 Dir(s)  24,485,937,152 bytes free

c:\@Work\Perl\monks\samira_saber>fc topol1.au topol1.top
Comparing files topol1.au and TOPOL1.TOP
FC: no differences encountered

c:\@Work\Perl\monks\samira_saber>type topol1.top
not so SOLITARY
SOLUTION
SOLVE the problem

c:\@Work\Perl\monks\samira_saber>perl process_file_1.pl topol1.top
processing 'topol1.top'

c:\@Work\Perl\monks\samira_saber>dir
 Volume in drive C is Acer
 Volume Serial Number is 9480-355B

 Directory of c:\@Work\Perl\monks\samira_saber

08/14/2017  05:26 PM    <DIR>          .
08/14/2017  05:26 PM    <DIR>          ..
08/14/2017  05:04 PM               759 process_file_1.pl
08/14/2017  04:40 PM                46 topol1.au
08/14/2017  05:26 PM                61 topol1.top
08/14/2017  04:40 PM                46 topol1.top.bak
               4 File(s)            912 bytes
               2 Dir(s)  24,485,937,152 bytes free

c:\@Work\Perl\monks\samira_saber>type topol1.top
not so SOL my new stuff
SOL my new stuff
SOL my new stuff

c:\@Work\Perl\monks\samira_saber>fc topol1.au topol1.top.bak
Comparing files topol1.au and TOPOL1.TOP.BAK
FC: no differences encountered
[download]