I don't think you absolutely need the DBD::Oracle line. DBI should load the correct module when you connect.

My personal solution has been to hide connection details in a module which always gets installed in The Right Place, has a home for tests and makes scripts clear and concise with subs like this

sub setup_environment {
    my $server = get_server_name();
    for my $env ( %{$environment{$server}} ) {
        $ENV{$env} = $environment{$server}->{$env};
    }
    return scalar keys %{$environment{$server}};
}
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I use it to cache database handles and such. It's not your perfect solution, but it works for me.