ilmari on IRC pointed me towards DBIx::Class::Storage::DBI::Oracle::Generic and the connect_call_datetime_setup method. While I can't say for certain that the on_connect_call option goes and finds the correct formatter, the docs say that it Does the Right Thing, i.e. it sets the necessary values and environment variables for DBIx::Class::InflateColumn::DateTime and DateTime::Format::Oracle. It just works.

There is a no-op sub in the base class DBIx::Class::Storage::DBI so that your code is portable across databases. The documentation on on_connect_call explains that on_connect_call => 'datetime_setup' calls DBIx::Class::Storage::DBI::Oracle::Generic->connect_call_datetime_setup() on Oracle databases which makes your InflateColumn call work.

some example code

my $schema = Timetable::Schema->connect("dbi:Oracle:$schema_name",
                        $db_username, $db_password,
                        {on_connect_call => 'datetime_setup'}
                );

my $resultset = $schema->resultset('Ical')
                       ->search($id);
print "Start time is ", $resultset->start_time;
# Start time is 2018-04-30T16:00:00
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use utf8;
package Timetable::Schema::Result::Ical;
use base 'DBIx::Class::Core';

__PACKAGE__->load_components("InflateColumn::DateTime");
__PACKAGE__->table_class("DBIx::Class::ResultSource::View");
__PACKAGE__->table("ICAL");
__PACKAGE__->add_columns(
  "id",
  { data_type => "varchar2", is_nullable => 0, size => 255 },
  "start_time",
  {
    data_type   => "datetime",
    is_nullable => 1,
    original    => { data_type => "date" },
  },
};

1;
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