I'm not entirely sure why it's persisting, but you do have an oddity in your code:

$test_hash{$group} = $group;

You're setting the value to a string, then in the for loops, treating it as a hashref. If you remove the line, or change it an empty hashref, you should be fine:

Update: I'm not sure if I should be proud for debugging the problem w/out using 'use strict', or feel like an idiot for not realizing it wasn't on the first place. I'm leaning towards the idiot side.