It isn't quite ignored, but it only really does anything if you're using pseudo hashes and you've defined your the class using use fields, in which case, $CGI would be initialized as an arrayref with an appropriate hashref in the zeroth slot.
Perl records this information, but does nothing (or
very little) with it.
Because it is recorded, the typesafety.pm and types.pm
modules are able to use it. This kind of core hooks
for extensibility via modules is a recurring pattern
in Perl: overload.pm can intercept the definitions of
constants and turn them into objects; attributes.pm
handles cases where tags like :foo appear after variables
or sub definitions (in cases where the tag isn't one of
the built ins like :lvalue, :method, or the depricated
:shared). This feature, like attributes, went through
growing pains. It is undocumented because it is still
experimental - the details of how and when it is used
are up in the air, even though some experimental modules
are using it now.
The short of it is, use types.pm and/or typesafety.pm.
has an intro to the idea of type safety and details about
the Perl implementations. It quotes this thread heavily =)
I hope this answers your question.