the colon depicts an (unrecognized) regexp extension

No, it doesn't. The colon is a literally matching atom, and the question mark is a quantifier. With some hopefully clearer /x spacing:

$foo =~ /( ( :? .          # 0 or 1 colon + 1 non-newline char
             |             # or
             \n (?! \n\n)  # 1 newline not followed by two more newlin
+es
            )+
          ) /x
[download]