sub bar() { -1 }
    $_ = -bar; #Ambigous ... as -&bar() ...
    print; # 1 !not "--1"
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Documented! From man perlop:

       Unary "-" performs arithmetic negation if the operand is
       numeric.  If the operand is an identifier, a string con-
       sisting of a minus sign concatenated with the identifier
       is returned.  Otherwise, if the string starts with a plus
       or minus, a string starting with the opposite sign is
       returned.  One effect of these rules is that "-bareword"
       is equivalent to "-bareword".

For fun, let "bar" return qq {-1}, and see one of the places in Perl were it matters whether a value is a string or a number.

Abigail