If you really want to do it in one unpack you can...

use strict;
use warnings;
use Data::Dumper;

my @arr = unpack('(W W @0Wx/a @1W@0Wx/x/a)', "\005\006Hello World");

print Dumper(\@arr);
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Which produces:

$VAR1 = [
          5,
          6,
          'Hello',
          ' World'
        ];
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But I would never do that, except to prove that I could.