This is guesswork, because I couldn't find much documentation on db ...

But it seems to be provided by the DBD (database driver) for the SQL server you use.

Those are normally implemented in XS resp. C, and their data structures can be magic.

Much like a Tie::Hash where FETCH is provided but not FIRSTKEY and NEXTKEY.

Consequently without inspection of the keys, one can't dump it.

For that one needs to know beforehand, which keys are accepted by FETCH.

update

To avoid confusion, this says that you are dealing with an object not a simple hash. To be precise a hash blessed into the package DBI::db. The rest holds.

1  DBI::db=HASH(0x33c6af0)
     empty hash
[download]

But you might want to check it again with Data::Dumper to rule out problems with the debuggers internal dumper.