Allow list assignment to list repetition

(undef,undef,undef,$foo,$bar)=that_function();

can now be written as

((undef)x3, $foo, $bar) = that_function();

Furthermore, (($a)x$assign_to_a,$b) = @c will include $a in the list
of variables ta assign to if $assign_to_a is 1, assign $c[0] to $b
if $assign_to_a is 0.  In other words, assuming $assign_to_a is 1 or
0, it is equivalent to:

($assign_to_a ? ($a, $b) : $b) = @c
perldata: Document list repetition assignment