ikegami & dave, thanks for your very insightful help! 8 )

it's clear now, that an extension of perl's inner mechanism of lvalue (which is a general one not only restricted to subs) can only be extended by tieing a callback function to the variable.

reading perlguts I found some slots to add external "magic"-properties to variables.

"The PERL_MAGIC_ext and PERL_MAGIC_uvar magic types are defined specifically for use by extensions and will not be used by perl itself."

IMHO this should be much faster than with classical oo-tie, but I'm not sure if there are other extension (*) relying on this mechanism which would interfere and break a "Turbo-Tie" solution ...

UPDATES:

* probably DBI ???