#!/usr/bin/perl -w
use strict;

while( my $data = <DATA> ) {
    chomp($data);
    next unless $data;
    my $fields = get_fields( $data );
    $fields->[rand(3)]->[0] = 'newval';
    my $str = remap_fields( $fields );
    print "$str\n";
}

sub get_fields {
    [ map{ s/^"|"$//g ? [ $_, 1 ] : [ $_, 0 ] } split /(?<=\S),(?=\S)/, $_[0] ];
}
sub remap_fields {
    join ',', map{ $_->[1] ? qq!"$_->[0]"! : $_->[0] } @{$_[0]};
}
__DATA__
"hi",3,20.6,"green","32"
16,"alpha",0.00