The cause of this new warning is withing the perl core itself, caused this commit.

It happens only when using threads and the boot code of DBD::Oracle is called twice.

Then the boot code tries to re-install the oracle specific methods and constants using DBI's internal install_method call.

You can indeed suppress that warning in DBI.pm, using something similar to

    my (undef, $filename, $line) = caller;
    # XXX reformat $attr as needed for _install_method
    my %attr = %{$attr || {}}; # copy so we can edit
    {   no warnings "redefine";
        DBI->_install_method ("DBI::${subtype}::$method", "$filename a
+t line $line", \%attr);
        }
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but that sweeps the warning under the carpet.

narrowing the scope, the warning is generated from the call cv = newXS_flags (meth_name, XS_DBI_dispatch, file)<c> in <c>DBI.xs in _install_method.

My personal opinion is that the boot code of DBD::Oracle is wrong, but at this moment, I have no idea of how to fix that.