"Permission Denied" occurs when you attempt to access a page which requires special permissions — permissions which you do not have.
Access controls on PerlMonks are implemented almost entirely by group membership.
Some nodes are accessible only to certain groups. For example, infrastructure code nodes are only accessible to members of the pmdev group.
(Of course, members of the gods group have access to all nodes. They are the site administrators, after all.)
Some nodes may have associated documentation which will be visible to all users, even if they cannot see the node proper.
One example of this would be expandfreenodelet, a fragment of infrastructure code.
You probably can't see the code (unless you're in pmdev or gods), but you can see the doclet which has been associated with the code.