use Data::Dumper;
# your initialization code here
print Dumper \%hash;

##</code><code>##

# WRONG
$hash{"LISTS"}=[([]) x $length]

##</code><code>##

$hash{LISTS} = [ map [], 1..$length ];

##</code><code>##

my %hash;

# automatically creates $hash{LISTS} as an array ref,
# and $hash{LISTS}[3] as an array ref:
push @{$hash{LISTS}[3]}, 'new item';

# also works for direct assignment:
$hash{LISTS}[4][0] = 'other new item';