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 (??{ code })
    ...
...
    subexpression may match. The result of evaluation is
    considered as a regular expression and matched as if
    it were inserted instead of this construct.

   1 while s!([BUI])(<+)\s*(.*?)\s*(??{'>' x length $2})!<$1>$3</$1>!;

   "B<<< I<< U<bold underlined text> >> >>>"

  <B><I><U>bold underlined text</U></I></B>

my $pod2html = qr/
  ([BUI])                  # start of formatting pattern
...
  \s*(.*?)\s*              # collect text, without border space
  (??{'>' x length($2)})   # match same number of closing braces
/x;