The error was actually hidden by the big eval{} in runworker, but with some debugging, one would have seen:

Storable::AUTOLOAD : attempting to run 'Bio::EnsEMBL::Hive::AnalysisJob::warning' (Bio::EnsEMBL::Hive::AnalysisJob=HASH(0x1ce1810), TEST, 1)
Storable::AUTOLOAD : could not parse 'warning' at /nfs/users/nfs_m/mm14/workspace/src/hive/master/modules/Bio/EnsEMBL/Hive/Storable.pm line 133.
        Bio::EnsEMBL::Hive::Storable::AUTOLOAD('Bio::EnsEMBL::Hive::AnalysisJob=HASH(0x1ce1810)', 'TEST', 1) called at /nfs/users/nfs_m/mm14/workspace/src/hive/master/modules/Bio/EnsEMBL/Hive/Worker.pm line 705
        Bio::EnsEMBL::Hive::Worker::run_one_batch('Bio::EnsEMBL::Hive::Worker=HASH(0x1c04078)', 'ARRAY(0x1cdced0)') called at /nfs/users/nfs_m/mm14/workspace/src/hive/master/modules/Bio/EnsEMBL/Hive/Worker.pm line 484
        Bio::EnsEMBL::Hive::Worker::run('Bio::EnsEMBL::Hive::Worker=HASH(0x1c04078)', 'HASH(0x1cbf510)') called at /nfs/users/nfs_m/mm14/workspace/src/hive/master/modules/Bio/EnsEMBL/Hive/Scripts/RunWorker.pm line 76
        Bio::EnsEMBL::Hive::Scripts::RunWorker::runWorker('Bio::EnsEMBL::Hive::DBSQL::DBAdaptor=HASH(0x119d618)', 'HASH(0x1af27b0)', 'HASH(0x1af2888)', 'HASH(0x1af2918)') called at /nfs/users/nfs_m/mm14/workspace/src/hive/master/scripts/runWorker.pl line 135
        main::main() called at /nfs/users/nfs_m/mm14/workspace/src/hive/master/scripts/runWorker.pl line 23