my $ds = {
    '986172' => { 'cat_parent_id' => '',       'cat_name' => 'Category1', },
    '986178' => { 'cat_parent_id' => '986177', 'cat_name' => 'Category4', },
    '986177' => { 'cat_parent_id' => '986176', 'cat_name' => 'Category3', },
    '986176' => { 'cat_parent_id' => '986172', 'cat_name' => 'Category2', }
};

# invert the structure:
my $sd = { map { $ds->{$_}{'cat_parent_id'} => { 'cat_child_id' => $_, 'cat_name' => $ds->{$_}{'cat_name'} } } keys %$ds };

sub foo  # recursive
{
	local $_ = shift || '';
	exists $sd->{$_} ? foo( $sd->{$_}{'cat_child_id'}, @_, $sd->{$_}{'cat_name'} ) : @_
}

print join ',', foo();