I get it! Keys are stringified. That's why. Ahhhh...
So, maybe what I could do is build an array of arrays:

my @samples =
(
   [  [ 1, 2, 3, 4 ], [ 5, 6, 7, 8 ] ],
   [  [ 1, 3, 5, 7 ], [ 2, 4, 6, 8 ] ]
);

my $sample;
foreach $sample (@samples)
{
   my @set = @$sample; 
   my @in  = @{$set[0]};
   my @out = @{$set[1]};
   
   print "in: @in  out: @out\n";
}
[download]

Or, if I really wanted to use the hashtable:

my %samples =
(
   'a' => [  [1, 2, 3, 4], [5, 6, 7, 8] ],
   'b' => [  [1, 3, 5, 7], [2, 4, 6, 8] ]
);

my $key;
foreach $key (keys %samples)
{
   my @set = @{$samples{$key}};
   my @in  = @{$set[0]};
   my @out = @{$set[1]};
   
   print "in: @in  out: @out\n";   
}
[download]

The second method allows me to label my samples.
How nice!

Thanks again.

Rob