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=back

=item cachenegative (0)

If true, even unsuccessful fetches from the underlying data are cached. This
can be useful to only search the underlying data once for each required key.

=item write (undef)

Provides a code reference to a routine which will write cache entries into the
underlying data. This routine will be called by write(), to synchronise the
underlying data with the cache. Called as C<$write-E<gt>($key,$val,$context)>.
If the routine is not provided, the underlying data will not be synchronised
after cache writes.

=item writethrough (1)

If true, the C<write> routine above will be called as soon as
write() is called. This provides immediate synchronisation of
underlying data and cache contents.

If false, the C<write> routine will
be called for each cache entry which no longer fits in its bucket after a
cache read or write. This provides a write-as-necessary behaviour, which may
be more efficient than the writethrough behaviour. However, only data fetched
through the cache will reflect these changes.

=item delete (undef)

Provides a code reference to a routine which will delete items from the
underlying data. This routine will be called by delete(),
to synchronise the underlying data with the cache. Called as
C<$delete-E<gt>($key,$cval,$context)>, where C<$cval> is the value
currently stored in the cache. If this routine is not provided, entries
deleted from the cache have no effect on the underlying data.

=back

An alternative to supplying a C<write> routine, is to call
delete() after updating the underlying data. Note however, that
in the case of databases, this should be done after committing the update, so
that a concurrent process doesn't reload the cache between being the entry