Insofar as I understand the (apparently edited) OP, a recursive approach also works (and is almost one line):

>perl -wMstrict -le
"my @a_refs = ([1], 2, [3, 4], [5, [6, [7, 8]]], 9);
 ;;
 sub flatten { map { 'ARRAY' eq ref() ? flatten(@$_) : $_ } @_ }
 ;;
 my @flattened = flatten(@a_refs);
 use Data::Dumper;
 print Dumper \@flattened;
"
$VAR1 = [
          1,
          2,
          3,
          4,
          5,
          6,
          7,
          8,
          9
        ];
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Note: This also works on the original example (Update (per GrandFather): as it was ultimately updated:)  ([1],[2],[3,4]) array.