An attribute list is a sequence of attribute specifications, separated
   by whitespace or a colon (with optional whitespace). Each attribute
   specification is a simple name, optionally followed by a parenthesised
   parameter list. If such a parameter list is present, it is scanned past
   as for the rules for the "q()" operator. (See "Quote and Quote-like
   Operators" in perlop.) The parameter list is passed as it was found,
   however, and not as per "q()".

   Some examples of syntactically valid attribute lists:

       switch(10,foo(7,3))  :  expensive
       Ugly('\(") :Bad
       _5x5
       locked method

   Some examples of syntactically invalid attribute lists (with
   annotation):

       switch(10,foo()             # ()-string not balanced
       Ugly('(')                   # ()-string not balanced
       5x5                         # "5x5" not a valid identifier
       Y2::north                   # "Y2::north" not a simple identifier
       foo + bar                   # "+" neither a colon nor whitespace