Flattening things appropriately was the bugaboo.

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

my $params = [ [ 2, 3, 4 ], 5, [ 6, 7, 8 ], 9 ];

sub get_numbers{
    my $array = shift;

    my ($car, @cdr) = @$array;
    ref $car or $car = [$car];
    @cdr or return $car;
    [ map {
    my $n = $_;
    map {[$n, (ref $_ ? @$_ : $_)] } @{get_numbers(\@cdr)};
    } @$car ];
}

print Dumper(get_numbers($params));
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