Since it sits on top of Ima::DBI you could always define a custom search routine.

MyClass->set_sql( null_somefield => <<'EOT' );
  SELECT __ESSENTIAL__
    FROM __TABLE__
    WHERE somefield IS NULL
EOT

my $iter = MyClass->search_null_somefield( );
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Update: Ooh, looks like passing undef as is suguested below works fine (it gets translated into field IS NULL correctly). Strange they don't explicitly mention that in the docs or on the Class::DBI wiki.