The perl manuals state that:

"The only situation where Perl's reference-based GC won't work is when there's a circularity in the data structure, such as:

$this->{WHATEVER} = $this;"

One can easily imagine such a self-reference. For instance, an object-oriented tax program might include a reference to one's employer. If one is self-employed then a self-reference could result.

It is still not immediately clear why Merlyn says that using eval in the situation he describes also creates a memory leak. I guess that the lesson for new monks in this thread is to avoid eval when it can be avoided, since it can have unknown deleterious effects on memory management.