fix parsing of braced subscript after parens

fix parsing of braced subscript after parens

Where an arrow is omitted between subscripts, if a parenthesised
subscript is followed by a braced one, PL_expect was getting set to
XBLOCK due to code intended for "foreach (...) {...}".  This broke
bareword autoquotation, and the parsing of operators following the
braced subscript.  Alter PL_expect from XBLOCK to XOPERATOR following
a parenthesised subscript.  Fixes [perl #8045].