What regular expressions did you try? Please post code, since you'll usually get better guidance in improving your programming skills that way.

Since there are backslashes involved more care is required to get good results. However, this can be implemented with something like:

#!/usr/bin/perl
use warnings;
use strict;

my $regex_1 = '\\\\'; # Need pairs because of escaping
my $regex_2 = '__BACKSLASH__';
for (<DATA>) {
    s/\Q$regex_1\E|\Q$regex_2\E/\\/g;
    print;
}

__DATA__
8\\.
8__BACKSLASH__.
[download]

Of particular note, I've used \Q and \E to escape regular expression metacharacters - omitting this is my guess on why you were having issues. See Quote and Quote like Operators in perlop for info on these.