I did a little CPAN-grep-ing and decided that $CLOSED was to generic and could potentially conflict, where $IS_CLOSED had no hits.

As for $ABSTRACT, I forgot that was once a common perl idiom (similar to $VERSION, etc), so best to move it to $IS_ABSTRACT instead (also no hits)

In general I think this will read better anyway, for instance:

if ($Foo::IS_CLOSED) { … } 

is nicer then …

if ($Foo::CLOSED) { … }

Where the later is what I think will be the more common usage style.