Returns true if the context of the currently executing subroutine or eval() block is looking for a list value. Returns false if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context).

    return unless defined wantarray;	# don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";

This function should have been named wantlist() instead.