#!/usr/bin/perl
use strict;
use warnings;

my @data;

open(DAT, '<', 'test.txt') or die $!;
while(<DAT>) {
  chomp;
  push @data, [ split( /,/ ) ];
}
close(DAT);

for my $row ( 0 .. $#data ) { # iterate over each row by index
  for my $column ( 0 .. $#{$data[$row]} ) { # iterate over each column by index
    print "row $row, column $column: $data[$row][$column]\n"; # display using the notation from the Arrow Rule
  }
}

# now let's just print the data in the 1st row, 1st column (zero based of course)
print $data[1][1],"\n";    # valid, see above
print $data[1]->[1],"\n";  # also valid, Rule 2
print ${$data[1]}[1],"\n"; # also valid, Rule 1