Good but seems not easily readable to me.

In that case, use a shorter string, associate the numbers from "Final program" against those on the right side , like 1: OPEN1 (3)

$ perl -Mre=debug -le " q/a a/ =~ /(.\b)\1/ "
Compiling REx "(.\b)\1"
Final program:
   1: OPEN1 (3)
   3:   REG_ANY (4)
   4:   BOUND (5)
   5: CLOSE1 (7)
   7: REF1 (9)
   9: END (0)
minlen 1
Matching REx "(.\b)\1" against "a a"
   0 <> <a a>                |  1:OPEN1(3)
   0 <> <a a>                |  3:REG_ANY(4)
   1 <a> < a>                |  4:BOUND(5)
   1 <a> < a>                |  5:CLOSE1(7)
   1 <a> < a>                |  7:REF1(9)
                                  failed...
   1 <a> < a>                |  1:OPEN1(3)
   1 <a> < a>                |  3:REG_ANY(4)
   2 <a > <a>                |  4:BOUND(5)
   2 <a > <a>                |  5:CLOSE1(7)
   2 <a > <a>                |  7:REF1(9)
                                  failed...
   2 <a > <a>                |  1:OPEN1(3)
   2 <a > <a>                |  3:REG_ANY(4)
   3 <a a> <>                |  4:BOUND(5)
   3 <a a> <>                |  5:CLOSE1(7)
   3 <a a> <>                |  7:REF1(9)
                                  failed...
   3 <a a> <>                |  1:OPEN1(3)
   3 <a a> <>                |  3:REG_ANY(4)
                                  failed...
Match failed
Freeing REx: "(.\b)\1"
[download]

Compare against a simpler pattern like

$ perl -Mre=debug -le " q/aa/ =~ /a\b/ "
Compiling REx "a\b"
Final program:
   1: EXACT <a> (3)
   3: BOUND (4)
   4: END (0)
anchored "a" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "a\b" against "aa"
Found anchored substr "a" at offset 0...
Guessed: match at offset 0
Matching REx "a\b" against "aa"
   0 <> <aa>                 |  1:EXACT <a>(3)
   1 <a> <a>                 |  3:BOUND(4)
                                  failed...
   1 <a> <a>                 |  1:EXACT <a>(3)
   2 <aa> <>                 |  3:BOUND(4)
   2 <aa> <>                 |  4:END(0)
Match successful!
Freeing REx: "a\b"
[download]

Then check the definition of \b in perlre#Assertions, perlrequick

Perl defines the following zero-width assertions: The word anchor \b matches a boundary between a word character and a non-word character \w\W or \W\w

     $x = "Housecat catenates house and cat";
    $x =~ /\bcat/; # matches cat in 'catenates'
    $x =~ /cat\b/; # matches cat in 'housecat'
    $x =~ /\bcat\b/; # matches 'cat' at end of string
[download]

Basically your pattern can never match, just like this perl -Mre=debug -le " q/aa/ =~ /a\ba/ "

there can never be a word boundary within a word by definition