Perl implements its garbage collection with a reference counting implementation. Every variable has a reference count associated with it. When you create a new referent to that variable, the reference count increases.

In usesfoo(), when you create a Foo object, you get $foo, with a reference count of one. (Yes, $foo is a referent.)

When you put $foo in the %ret hash, all of a sudden $ret{foo} becomes a referent to that to which $foo points. (No dangling participle here.) Reference count++. That makes it two.

At the end of the subroutine, after the return, the data structure we'll call %ret is still around because it gets returned. $foo is not so lucky, and goes out of scope. Reference count--. That makes it one. So whatever $foo pointed to is still around because what we call %ret in the subroutine still points to it.

When you get rid of %ret, Perl will go through it all and decrease all the reference counts. Assuming that the rc for $foo is one at that point, it'll get garbage collected.

Adding the DESTROY method to the flyweight class was a good idea. The memory leak you speak of would be in the block lexical @foos, if anywhere.

Anyone in the know who's just stopped reeling from my simplification is welcome to jump in here and correct me.