Multidimensional Hash Elements in Perl6

<> is the same as {''} in perl5 so should only be used for string keys. If you want to use the variable names go with :

%key{$barcode}{$position} = $sample_id this should do the trick.

You can also use the double pointy brace option which does string interpolation.

%key<<$barcode>><<$position>> = $sample_id