Or more generally: a way to invert a hash (keys become values and vice versa) without loosing any data:

use strict;
use warnings;
use Data::Dumper;

my %hash;
while (<DATA>) {
    my ( $key, $value ) = split;
    $hash{$key} = $value;
} ## end while (<DATA>)
print "Original hash\n", Dumper( \%hash );

my %data_structure;
foreach my $key ( keys %hash ) {
    push @{ $data_structure{ $hash{$key} } }, $key;
} ## end foreach my $key ( keys %hash)
print "Inverted hash\n", Dumper( \%data_structure );

__DATA__
key1, value1
key2, value2
key3, value1
key4, value3
key5, value2
key6, value4
key7, value4
[download]

output

Original hash
$VAR1 = {
          'key3,' => 'value1',
          'key2,' => 'value2',
          'key4,' => 'value3',
          'key6,' => 'value4',
          'key7,' => 'value4',
          'key5,' => 'value2',
          'key1,' => 'value1'
        };
Inverted hash
$VAR1 = {
          'value4' => [
                        'key6,',
                        'key7,'
                      ],
          'value1' => [
                        'key3,',
                        'key1,'
                      ],
          'value3' => [
                        'key4,'
                      ],
          'value2' => [
                        'key2,',
                        'key5,'
                      ]
        };
[download]

The resulting data-structure is a Hash-of-Arrays.