In Perl it is possible to instantiate multiple variables like so:

my ($a, $b, $c) = (1,2,3);

It is also possible to reassign multiple variable values the same way:

($a, $b, $c) = (4,5,6);

However, when I try to do the same thing with the plus equals operator,

($a, $b, $c) += (7,8,9);

only $c is properly added and the other variables remain their original value. Is this something that should be possible in Perl, or is it just partially working by accident and it really doesn't work that way? If the latter is true, is there a way to to this in one line?