Not the most elegant, but this does the job:

$_=join qq#\\\\#,split(/\\/,$_,-1);
[download]

As far as shedding light on your code:
The problem is that the "g" tag, by itself does not repeat replacement.

You need to put the substitution into a loop until no more substitutions are made.

The problem with that approach is that (for me) that loops infinitely, as it discovers more "\\" replacing "\".

I'm sure a wiser monk will provide the appropriate elegant hack.

Here is another approach, using substitution:

>perl -e "my @s=qw(\\\\c\\c\\ d\\d aa bb); for my $x(@s){($y=$x)=~ s#(
+\\+)#$1$1#g;print qq{$y=$x;\n}}"
\\\\c\\c\\=\\c\c\;
d\\d=d\d;
aa=aa;
bb=bb;
[download]

Update:Struck incorrect statements, and improved second sample code.