At the top of that sub where $self is declared and assigned, put in confess unless ref($self) eq 'NameOfObjectHere'  (use Carp if you aren't already)

That will give you the trace dump and you can see what code is passing an unblessed reference in. Follow back to the source of that unblessed value until you find where it is created.

I suspect some part of the code isn't checking for errors before using the reference, and just assuming it worked. Some part of your environment causes the original creation and/or blessing to fail and you need to throw a message about why when it happens.