thank you for your reply ... Yes that would be the proper way of doing it, you are absolutely right and I will look into it. For now , here is a quick and dirty solution that solved it for me:

use strict;
use warnings;
use File::Basename;
use Text::ParseWords;

if ($#ARGV == 0) {

        open my $file_in, "<", $ARGV[0] or die "Couldn't open file '$A
+RGV[0]': $! \nDid you specify a valid file?";
    # open up a new file to write the changes made to
    open my $file_out, ">", "$ARGV[0].new" or die "Can't write new fil
+e  '$ARGV[0].new' : $! \nDo you have write permissions?"; 

    # these are our currently active market makers
    my @list=("BNP","CBK","CIT","NDS","OHD","OHM","RBN","RBS","SEK","S
+GA");
 
        while (<$file_in>) {
        ## write all changes to new file
        print $file_out $_;
        # if we find a match for any Symbols
        if ($_ =~ m%</Name><Symbol>([^<]+)%) {
            my $SYMBOL;
            my $MATCH;
            $SYMBOL = $1;
        # and the $SYMBOL matches the array @list for active market ma
+kers
            if (grep {$_ eq $SYMBOL} @list) { 
        # Print and add the line marketMakerOrganization: $SYMBOL to t
+he $file_out
                print $file_out "\t\tmarketMakerOrganization:\"$SYMBOL
+\",\n";
                                 }
                                  }    
            }
} else {
        print "You need to specify an input file \n";
        print "\n";
        print "They are usually located here : \n";
        print "/PATH/orderbooks-xx-hostname-yy.x.xxx.xx.js";
        print "\n";
        print "Usage : ".basename($0)." difffile.txt \n";
        print "\n";
        exit;
}
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