The problem is with your understanding of the data structure and the value of $heap. If you output the data structure you actually have in $heap, maybe that will show where the error is. Use Data::Dumper to look at the data structure:

use Data::Dumper;
warn Dumper $heap;
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You can help us help you better by providing a short, self-contained example that fails. Also, telling us the exact error message and the exact error location in your code provides us with more hints. There is a reason why Perl outputs them, but you chose to hide them from us.