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 }