Using ad_export from DBD::AnyData you can convert in-memory databases into a file of specified format, including DBI databases. You can also pump out a host of other formats too.

This may require some re-organising of your datastructure though? Possibly a silly thought, possibly not, but i am not sure how compliant Berkley DB and DBI are... Although by the looks of it, DBD::AnyData seems to support many database formats. HTH.