You seem to want a very arbitrary structure, but here's one way for the given example data:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @array = ('a', 1, 2, 3, 4, 'b', 6, 7, 8);
 ;;
 my %hash = (
   $array[0] => [ @array[ 1 .. 4 ] ],
   $array[5] => [ @array[ 6 .. 8 ] ],
   );
 ;;
 dd \%hash;
"
{ a => [1, 2, 3, 4], b => [6, 7, 8] }
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Can you be more specific about what you really want?