Data::Dumper is going to give out "DUMMY" as the value for any sub, not just a closure. The contents of a subroutine are represented internally with a variety byte code I believe, which would look like so much nonsense if it were printed in the contents of a Data::Dumper dump. Contrast to scalars (or hash keys or hash/array values made from scalars) and to references. Scalars eventually turn out to be some sequence of characters, and references inside a data structure can be represented with brackets and braces.
has more info on this in the BUGS section.