13-th (xmath says that this is a cheat)

use subs 'and';sub and :lvalue {$_};

9-th

$foo="\n0";$*++;

7-th

*foo = \1;

6-th

sub f{\@_}; *foo = f($_,$_);

2-d

*foo=\pos;

1-st

$foo = *_;

Update: 2-nd and 9-th added

Update: 10-th REMOVED, because it use the same method as 9-th