initially at parse time a goto-expression is free of special cases, so goto &foo is just a goto-element with the ENTERSUB expression &foo as argument. Now, the "goto SUB" variation needs a subref ofcourse, and to accomplish this perl effectively puts a \ in front of the argument, turning goto &foo into goto \&foo. Now here comes the problem: it does this whenever the argument is an ENTERSUB, and forgets to check for the ampersand (!(o->op_flags & OPf_STACKED)), so goto +foo becomes goto \foo, which results in the strange error message (the +-prefix is already dropped at parse time, and is just to prevent the special-case handling for goto LABEL to be triggered)

Questions? :-)