$  perl -MYAPE::Regex::Explain -le'die YAPE::Regex::Explain->new(qr/(\b\w+)\1\b/)->explain'
The regular expression:

(?-imsx:(\b\w+)\1\b)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    \b                       the boundary between a word char (\w)
                             and something that is not a word char
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \1                       what was matched by capture \1
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------