I assume you're not continually adding to $data ... That's way too obvious.

Besides the note already mentioned about eval leaks in older versions of Perl, the other big one to watch out for is self-referential data; i.e., references that end up pointing back to themselves. That's usually not obvious, as in a directed graph type data structure where a node may end up pointing back to one of its ancestors. I had one of those that blew a Perl program of mine up in a major way. Once identified you can use WeakRef to fix them.