Using persistent private variables in xs

[Jeremy White]

Given this code:

{
   my $variable;
   sub mysub {
       # ... accessing $variable
   }
}
from:

http://www.unix.org.ua/orelly/perl/cookbook/ch10_04.htm

Is there a way to get hold of the SV for $variable and manipulate it via XS?

I want to be able to write the contents of $variable in XS, then call mysub 
via XS and then read the contents of $variable afterwards ($variable needs 
to remain a closure as it is shared with other subs in the same block - the 
block is evaled into existence during runtime).

All my attempts so far have failed, so I'm either being stupid, or can't it 
be done?

Cheers,

jez.