[Catalyst] Suddenly failing with MOP errors

[Nick]

After some days of running without incident my catalyst web site
started failing with

Use of uninitialized value in subroutine entry at /usr/lib/perl5/Class/MOP/Package.pm line 243.
Use of uninitialized value in subroutine entry at /usr/lib/perl5/Class/MOP/Package.pm line 243.
Use of uninitialized value in subroutine entry at /usr/lib/perl5/Class/MOP/Package.pm line 243.
Use of uninitialized value in subroutine entry at /usr/lib/perl5/Class/MOP/Class.pm line 496.
[error] Caught exception in engine "Can't call method "isa" on an undefined value at /usr/lib/perl5/Class/MOP/Class.pm line 496."

the same sequence appearing repeatedly in error.log and with a blank
page on the browser.

The nearest case I can find is this
http://www.mail-archive.com/catalyst@lists.scsys.co.uk/msg10990.html
but I am not using NonMoose.

I restarted Apache and the problem has not recurred... yet.

Help?
-- 
Nick