my $any_slash_or_at = qr{ [\\@] }x;

##</code><code>##

s/(?<![\\\@])[\\\@]\@/\x11/g;
s/(?<![\\\@])[\\\@]\\/\x12/g;

##</code><code>##

s/$slash_or_at__at/DC1/g;
s/$slash_or_at__slash/DC2/g;

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my $test_string
    = '\\\\\\ \\\\@ \\@\\ \\@@ @\\\\ @\\@ @@\\ @@@ \\\\ \\@ @\\ @@ \\ @';

my $any_slash_or_at = qr{ [\\@] }x;
my $lone_slash_or_at = qr{ (?<!$any_slash_or_at) $any_slash_or_at }x;
my $slash_or_at__at = qr{ $lone_slash_or_at \@ }x;
my $slash_or_at__slash = qr{ $lone_slash_or_at \\ }x;

# Your code - a lot easier to read

print 'Testing your code:';
$_ = $test_string;
print;
s/$slash_or_at__at/DC1/g;
s/$slash_or_at__slash/DC2/g;
print;

# An alternative with a less complex regex and a single substitution

my %replace = ( '@' => 'DC1', '\\' => 'DC2' );
my $escape_slash_or_at = qr{ $any_slash_or_at ( $any_slash_or_at ) }x;

print 'Testing simultaneous replacements:';
$_ = $test_string;
print;
s/$escape_slash_or_at/$replace{$1}/g;
print;

##</code><code>##

Testing your code:
\\\ \\@ \@\ \@@ @\\ @\@ @@\ @@@ \\ \@ @\ @@ \ @
DC2\ DC2@ DC1\ DC1@ DC2\ DC2@ DC1\ DC1@ DC2 DC1 DC2 DC1 \ @
Testing simultaneous replacements:
\\\ \\@ \@\ \@@ @\\ @\@ @@\ @@@ \\ \@ @\ @@ \ @
DC2\ DC2@ DC1\ DC1@ DC2\ DC2@ DC1\ DC1@ DC2 DC1 DC2 DC1 \ @