gistfile1.txt

┌─[tadzik@yavin]─[~] 
└─[%]─> cat json-bug.pl 
my $json = '{"foo":[]}';
my ($tiny, $fast);
{
    use JSON::Tiny;
    $tiny = from-json $json;
}
{
    use JSON::Fast;
    $fast = from-json $json;
}

say $tiny.perl;
say $fast.perl;
┌─[tadzik@yavin]─[~] 
└─[%]─> perl6 json-bug.pl 
{:foo($[])}
{:foo($[])}