my %hash = (
	'key1' => [1, 20, 3],
	'key2' => [1, 30, 4, 7],
	'key3' => [1, 9, 8],
	'key4' => [1, 9, 17],
	'key5' => [1, 9, 17],
);

my %rev_hash;

while(my ($k, $v) = each %hash) {
	push @{$rev_hash{$_}}, $k for @$v;
}