use re 'debug';
$x = "A\\B";
$y = "Here is A\\B";
print $y =~ /$x/ ? "y" : "n";
__END__
Compiling REx `A\B'
size 4 Got 36 bytes for offset annotations.
first at 1
   1: EXACT <A>(3)
   3: NBOUND(4)
   4: END(0)
anchored "A" at 0 (checking anchored) minlen 1
Offsets: [4]
        1[1] 0[0] 2[2] 4[0]
Guessing start of match, REx "A\B" against "Here is A\B"...
Found anchored substr "A" at offset 8...
Starting position does not contradict /^/m...
Guessed: match at offset 8
Matching REx "A\B" against "A\B"
  Setting an EVAL scope, savestack=3
   8 <Here is > <A\B>     |  1:  EXACT <A>
   9 <Here is A> <\B>     |  3:  NBOUND
                            failed...
Match failed
nFreeing REx: `"A\\B"'