#!/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__.