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