I strongly recommend you use Text::CSV for CSV input/output, your script will end up much more robust and probably a bit shorter (also install Text::CSV_XS for speed). I gave an example of CSV input and output here. Update: Copied over code example (with minor adaptations):

use warnings;
use strict;
use Text::CSV;

my $csv = Text::CSV->new({ binary=>1, auto_diag=>2,
    eol=>$/, sep_char => ";" });
open my $ifh, '<', 'Input.CSV' or die $!;
open my $ofh, '>', 'Output.CSV' or die $!;
while ( my $row = $csv->getline($ifh) ) {
    
    # your logic to modify data here
    
    $csv->print($ofh, $row);
}
$csv->eof or $csv->error_diag;
close $ifh;
close $ofh;
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