Sometimes, the line number in error messages are off by a bit. I think that's the case here.

undef @uniq{@dist_list};
is equivalent or similar to
$uniq{$_} = undef for @dist_list;

Together, the three lines remove duplicate items in @dist_list, assuming @dist_list only contains strings. (Anything in @dist_list which isn't a string is first converted to a string.) The order of the items in @dist_list is not guaranteed to remain unchanged.

By the way, it's <code>..</code> (or <c>..</c>), not <code>..<code/>.