Function pointer    Action taken
    ----------------    ------------
    svt_get             Do something after the value of the SV is retrieved.
    svt_set             Do something after the SV is assigned a value.
    svt_len             Report on the SV's length.
    svt_clear           Clear something the SV represents.
    svt_free            Free any extra storage associated with the SV.

##</code><code>##

 attach ( $somepointer, { seen=>1 }, 'opt_name_to_avoid_collision') or die "Couldn't attach;
 # Later on...
 my $attached = attach($somepointer);
 $attached->{bar}= uc $attached->{bar};