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().
puting () after shift 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;

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or something like 
shift ?pattern? ? @a: @b;

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the parentheses forces the first interpretation and shift defaults to using @_.

In reply to Re: Use of shift in Ternary Operator by hipowls
in thread Use of shift in Ternary Operator by cheruvim

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