Further to runrig's replies:
It's important to understand at each step how the structures of the hash and arrays are changing and evolving. Here's a kind of "running commentary" on that evolution, and a possible way to access and print what you want. Liberal use of data structure (for that's what you're dealing with; see perldsc) dump statements can be very enlightening about the changes that are going on. (And BTW, congratulations on being able to work with a system having at least 128 Terabytes of system RAM — or do I misinterpret the reference address of 0x7ffafc013bf8?)
>perl -wMstrict -MData::Dump -le
"my %mapp;
my @arr = qw(R T HH M);
;;
$mapp{k} = \@arr;
my @val = $mapp{k};
;;
dd \%mapp;
dd \@val;
;;
print join q{,}, @{$mapp{k}};
;;
push @val, 'RBB';
dd \@val;
;;
$mapp{k} = \@val;
dd \%mapp;
;;
print join q{,}, @{$mapp{k}};
print join q{,}, @{ $mapp{k}->[0] }, $mapp{k}->[1];
"
{ k => ["R", "T", "HH", "M"] }
[["R", "T", "HH", "M"]]
R,T,HH,M
[["R", "T", "HH", "M"], "RBB"]
{ k => [["R", "T", "HH", "M"], "RBB"] }
ARRAY(0x1e14b3c),RBB
R,T,HH,M,RBB
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