Hi Lakssreedhar
Use a 2D array to read a file column wise. Then adopt the folowing steps :

1. count the number of lines of your file.
2. Use some regular expression to split the file with respect to the column delimiters i.e tab in your case . Thus build the 2D array.
3. Use a sub routine to get all the values of the 9th column and push them in an array.
4. Process the array to look where at which index I is preceded by O. Find those indexes.
5. open your data file again and using the 2D matrix just replace those positions .
The following code could be a bit help to you

my @matrix;
my $row = 0;

# count no. of lines 
open(DATA, "< data.txt");
my @lines = <DATA>;
close(DATA);
my $c = @lines;

open(DATA, "< data.txt");

push @{$matrix[$row++]}, split while <DATA>;
dd @matrix;




&colprint(\@matrix);


close(DATA);
my @arrayOfninth_col;
sub colprint
{
    my $matrix_ref = shift;
    for (1 .. $c)
    {
        print "$matrix_ref->[$c-1][9] \n";
                my $value = $matrix_ref->[$c-1][9];
                push @arrayOfninth_col , $value;


    }

   # process the array @arrayOfninth_col for indexes 
}
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i have just split the columns using tab and then again split the 9th column and checked if value of a particular index is I and index-1 is o.But the substitution is not working properly

which substitution, paste your code here