... explain the sub chain{} function ...

The"Perl Magick" here is recursion; it is not peculiar to Perl in any way. 

sub chain {
    return @_ > 1 ? { shift() => chain(@_) } : shift();
    }

[download]
If there are two or more items in the @_ function argument array (@_ > 1), { shift() => chain(@_) } executes. shift takes the top argument off of @_ and makes it a hash key with the value of whatever the call to chain(@_) returns. Note that chain(@_) is called with whatever remains in the @_ array after one item has been removed by the shift call. This key/value pair is then returned within an anonymous hash reference.

If there are fewer than two items in the @_ array, the call to chain(@_) simply returns the top item in the array. Note that this does not "properly" handle the case in which the @_ array is empty. What is "proper" handling in this case (if it can even arise)? Only you can figure this out. 

Win8 Strawberry 5.8.9.5 (32)  Thu 05/13/2021 15:26:20
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

my %hash;

# my ( $top, @rest ) = split /,|\n/; # FIXME faking CSV for testing
my ($top, @rest) = qw(foo);

$hash{$top} = chain(@rest);

use Data::Dump 'dd'; dd \%hash;

sub chain {
    return @_ > 1 ? { shift() => chain(@_) } : shift();
    }

^Z
{ foo => undef }

[download]

... remove the first row so the header doesn't do what the rest of the hash is doing.

One might do the following:
    open(my $input, '<:utf8',"input.csv") or die;

    <$input>;  # read and discard one line/record
(I think Text::CSV can do this for you (it does just about everything else :), but you'll have to check for yourself.)

Give a man a fish:  <%-{-{-{-<

In reply to Re^3: Creating a Multi Level Hash from CSV by AnomalousMonk
in thread Creating a Multi Level Hash from CSV by workInProgress12

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