The problem is that ? is ambiguous, is it part of a ternery operator or the start of a ?? match? From Perl Regular Expressions Reference
?pattern? is like m/pattern/ but matches only once. No alternate delimiters can be used. Must be reset with reset().
resolves the ambiguity.
Update:To explain further, shift expects a single argument which is an expression that evaluates to an array. If no argument is provided it defaults to @_. When perl sees shift ? it doesn't know if you mean
shift(@_) ? 1: 2;
or something like
shift ?pattern? ? @a: @b;
the parentheses forces the first interpretation and shift defaults to using @_