Families of Variable Names

        $chain[$chain_number] = [split //, $chain_input];
        $scan_chain_length[$chain_number] = @{$chain[$chain_number]};
        &printout if $chain_number == 7;

We can trim this a little more
Only $scan_chain_length[3] is ever used
We'll just write @{$chain[3]} when we need it

        $chain[$chain_number] = [split //, $chain_input];
        &printout if $chain_number == 7;