How to append new values to hash in Perl

Question

I have a hash of hashes where, at the last level, I want each value to be appended - not updated - if that value already exists. What would be the best way to do this? I was thinking about making the values as lists, but this is probably not the most efficient way...

Here's where I got so far:

#!/usr/local/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $dir='D:\';

open my $data,"<","$dir\\file.txt";

my (@selecteddata,@array,%hash);

while (<$data>) {
    chomp $_;
    my @line= split "\t";

    $hash{$line[1]}{$line[2]}=$line[0]; 
    warn Dumper \%hash;
}
close $data;

Note, this code updates the values at last level with value $line[0], but if the key $line[4] already exists (meaning, it already has a previous value $line[0]) I want this value to be appended and not updated.

So, ideally, for the following (tab sepparated) list:

a1  b1  c1
a2  b2  c2
a3  b3  c3
a4  b4  c4
a5  b4  c4

The hash would look something like this - I don't know exactly how the grouping of a4 and a5 should look like, so as long as they are grouped it should be ok:

{
    'b1'    =>  {'c1'   =>  'a1'},
    'b2'    =>  {'c2'   =>  'a2'},
    'b3'    =>  {'c3'   =>  'a3'},
    'b4'    =>  {'c4'   =>  'a4, a5'}
}

Answer 1

You can append your string,

$_ = defined($_) ? "$_, $line[0]" : $line[0] 
  for $hash{$line[1]}{$line[2]};

or use array which is better suited for storing list of elements,

push @{ $hash{$line[1]}{$line[2]} }, $line[0];