I see a problem with: @row=('aa','bb','\\c\c\','d\d');

I believe the compiler is reading an odd number of tick marks (') because your example is escaping the tick mark that follows the last c\ combination.

When I read @row as input (see below), I get the same output you showed for "result". I assumed that was your desired output.

my @row=(<DATA>);
my $separator=',';
foreach (@row) {
  s/(\\+)/$1$1/g;
}
print join($separator, @row);
print "\n";

__DATA__
aa,bb,\\c\c\,d\d
[download]

Output:
aa,bb,\\\\c\\c\\,d\\d
[download]