*foo = *bar;
*foo = Symbol::gensym
*foo = \*bar;
local *foo;

##</code><code>##

%pkg:: = (foo => *bar, %pkg::);

##</code><code>##

package pkg;
$save_old_foo = \*foo;
delete $pkg::{foo};
*{"pkg::foo"} or 0; # make perl create a new glob
...code using replaced glob goes here...
delete $pkg::{foo};
*pkg::foo = $save_old_foo;