my %hash = (jan => [january, '01'], feb => [february, '02'], mar => [march, '03']);

walk_data_struct (\%hash, sub { my $leaf = shift; print "$leaf \n";});

sub walk_data_struct {
	my ($data_ref, $code_ref) = @_;
	if (ref $data_ref) {
		if (ref ($data_ref) eq "HASH") {
			walk_data_struct ($data_ref->{$_}, $code_ref) foreach keys %$data_ref;
		} else {
			walk_data_struct ($_, $code_ref) foreach @$data_ref ;
		}
	} else {
		&{$code_ref} ($data_ref);
	}
}