#!/usr/bin/env perl
use warnings;
use strict;
use Text::CSV;

my $csv = Text::CSV->new({ binary=>1, auto_diag=>2,
	eol=>$/, always_quote=>1 });
open my $ifh, '<', 'NRoomch.txt' or die $!;
open my $ofh, '>', 'output.txt' or die $!;
while ( my $row = $csv->getline($ifh) ) {
	
	$row->[1]='P'; # your logic to modify 2nd column here
	
	$csv->print($ofh, $row);
}
$csv->eof or $csv->error_diag;
close $ifh;
close $ofh;