The reason is the anchors ^ and $. As revealed by YAPE::Regex::Explain:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  B*                       'B' (0 or more times (matching the most
                           amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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In order for a match to occur, all characters between the start and end of the string must be 'B' (with the possible addition of a trailing newline). See perlretut.