my @sorted = ('A'..'Z'); # probably sort keys %some_hash;

reduce(@sorted);

sub reduce {
    my @ten = splice(@_, 0, 10); 

    return if (!@ten);       # Base case
    # return if (@ten < 10); # Or do you want to stop if < 10 elements?
    
    print join(',',@ten) . "\n";

    return reduce(@_); # Recursion case, remainder of array
}