I think the symbol gets garbage collected on newer Perls, but on older ones, you must explicitly ungensym it.

Update: Yes, here's what it says:

from perldoc Symbol:

"Symbol::gensym" creates an anonymous glob and returns a reference to it. Such a glob reference can be used as a file or directory handle.

For backward compatibility with older implementations that didn't support anonymous globs, "Symbol::ungensym" is also provided. But it doesn't do anything.