You can take one of two approaches:

1/ edit the string, probably using a regex

2/ split the string into an array and edit the elements

We can't tell you what will work best because you don't tell us enough about the overall process. You asked in essence "how do I edit elements of an array" and got good answers. Now you show us a process involving reading a line, spliting it into an array, editing the array, ... . Then you tell us you don't want to do that but instead want to do something else! What do you want to do?

I suggest you try several approaches on a small subset of your data and benchmark them to see what works best. The following may be a good starting point:

use strict;  
use warnings;
use Benchmark qw(cmpthese);

my $line = join (',', 1 .. 1000) . "\n";
my $data = $line x 100;

cmpthese (
    -1,
    {
        array => \&asArray,
        str   => \&asStr,
    }
);


sub asArray {
    my $outStr;
    
    open my $in, '<', \$data;
    open my $out, '>', \$outStr;
    
    print $out join ',', map {s/$/a/; $_} split ',' while <$in>;
    
    return $outStr;
}


sub asStr {
    my $outStr;
    
    open my $in, '<', \$data;
    open my $out, '>', \$outStr;
    
    while (<$in>) {
        s/(?=,)|\n/a/g if /\S/;
        print $out $_;
    }
    
    return $outStr;
}
[download]

Prints:

        Rate array   str
array 6.23/s    --  -28%
str   8.61/s   38%    --
[download]

which indicates that the differences between the two techniques are small enough that they would probably be completely lost in the I/O time.

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