# @c = grep !defined, @c;
# is O(N) speed, O(N) memory.
#
# The following does the same, but
# is O(N) speed, O(1) memory.

my $last = -1;
my $i    = -1;
while (++$i < @c) {
   if (defined($c[$i]) {
      $c[++$last] = $c[$i];
   }
}

$#c = $last;

##</code><code>##

pop @c while $#c > $last;