The Mysterious Immutable Mu

#!/usr/bin/env perl6

use v6;

my $n = 30;

proto deepclone($) {*}
multi deepclone(%hash is copy) {
    say "hash";
    %hash{$n++} = 1;
    # there are problems with hyper smartmatching right now
    #%hash = %hash.map( { $_.key => ( ( [||] $_.value <<~~<< ( Positional, Associative ) ) ?? deepclone( $_.value ) !! $_.value ) } );
    my %hash2 = %hash.map( { $_.key => ( ( $_.value ~~ Positional || $_.value ~~ Associative ) ?? deepclone( $_.value ) !! $_.value ) } );
    say "/hash";
    %hash2;
}
multi deepclone(@array is copy) {
    say "array";
    @array.push( $n++ );
    # there are problems with hyper smartmatching right now
    #@array = @array.map( { deepclone( $_ ) if [||] $_ <<~~<< ( Positional, Associative ) } );
    my @array2 = @array.map( { ( $_ ~~ Positional || $_ ~~ Associative ) ?? deepclone( $_ ) !! $_ } );
    say "/array";
    @array2;
}
multi deepclone($unknown is copy) {
    say "unknown";
    $unknown;
}

my $h = { a => 1, b => [ 2, 3, { c => 4, d => 5 } ] };

my $i = deepclone( $h );

say $h;
say $i;

$i{'b'}[2]{'e'} = 6;
say "mark a";
$i{'b'}.unshift( 0 );
say "mark b";
$i{'e'} = 7;

say $h;
say $i;

=begin output
hash
array
hash
/hash
/array
/hash
"a" => 1, "b" => [2, 3, {"c" => 4, "d" => 5}]
"30" => 1, "a" => 1, "b" => [2, 3, "c" => 4, "d" => 5, "32" => 1, 31]
Cannot modify an immutable Mu
  in method assign_key at src/gen/m-CORE.setting:1968
  in sub postcircumfix:<{ }> at src/gen/m-CORE.setting:2902
  in block <unit> at t1.p6:38

=end output