The $tsh is dereferenced to populate %br_tsh and it's a form of copying as well. Meaning, further change on %br_tsh, for example, $br_tsh{some_key} = 'newvalue' won't affect $tsh->{some_key} (assuming the key exists).

If begin_time and end_time exist in $tsh, they are both overridden in %br_tsh due to explicit assignments. Any later update on %br_tsh doesn't affect $tsh.

HTH,