push @return, $self->_rmap($_) for values %$_;
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Hmm, if you do this another way, you must be able to avoid the problem:

push @return, map { $self->_rmap($_) } values %$_;
[download]

Here, values is finished before the map even starts.

However... you won't have a problem in the original unless the values is traversed lazily. You never know...