First of all, as it was pointed above, you should either create a reference to a hash (my $reference = { ... }) or just a hash (my %hash = ( ... )), note the brackets.

Next, contents of hash should be accessed using curly, not square brackets (the second is also possible because hashes can be treated as arrays). Example:

my %entry = ($item_id=>\@data, $item_id=>\@data);
print $entry{$i}->[$j]; # contents of the hash ({}) are array referenc
+es; we use -> to work with references; then we use [] to work with ar
+rays
# OR:
my $reference = {$item_id=>\@data, $item_id=>\@data};
print $entry->{$i}->[$j];
# firstly, $entry is a reference to a hash, use ->
# secondly, hash contents are accessed using {}
# thirdly, it's again a reference
# fourthly, arrays are accessed using []
[download]

Next, there is a special syntax to treat a variable as a reference: you can always use an array reference, in curly braces, in place of the name of an array. To get the index of an array by its reference, write $# and then write the reference in curly brackets:

my $index = $#{$entry{$i}};
[download]

See perlreftut for more.