Worse, it's the new extended kind which returns key-value pairs, forgot the terminology for it °

  DB<23> $hr= { To => [qw/a b c/] }

  DB<24> x %$hr{To}
0  'To'
1  ARRAY(0x55ef21ab6818)
   0  'a'
   1  'b'
   2  'c'
  DB<25> x @$hr{To}              # classic hash slice
0  ARRAY(0x55ef21ab6818)
   0  'a'
   1  'b'
   2  'c'
  DB<26> x @{%$hr{To}}           # only "works" with one hash key 
0  'a'
1  'b'
2  'c'
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update

interestingly even the documentation of this module is faulty

https://metacpan.org/dist/Mail-IMAPClient/view/lib/Mail/IMAPClient.pod#parse_headers

    "To"      => [ "Big Shot <big.shot@bigco.com> ] ,

(spot the missing " )

I think the OP should use Data::Dumper or Data::Dump to inspect the $hashref and isolate his problem.

°) apparently key-value-slice since the "new" (cough) 5.20 ... wait... perldoc says Key/Value-Hash-Slice