If you'd enabled strict you'd have gotten a complaint that you can't use the string "group1" as a HASH ref. The problem is that your line initializing $test_hash{ $group } = $group prevents perl from autovivifying that as a hashref later when you try and use it as such in the for loop. What's happening is that you've got a symbolic reference to the hash %{ $test_hash{ $group } } (i.e. %main::group1) and that's persisting between the two calls (since it's not a lexical).

Either don't initialize $test_hash{ $group }, or explicitly initialize it with a hashref $test_hash{ $group } = {}.