Using a module is usually a good idea. But for fun, here's my hand-rolled solution.

my $ds = {
    '986172' => { 'cat_parent_id' => '',       'cat_name' => 'Category
+1', },
    '986178' => { 'cat_parent_id' => '986177', 'cat_name' => 'Category
+4', },
    '986177' => { 'cat_parent_id' => '986176', 'cat_name' => 'Category
+3', },
    '986176' => { 'cat_parent_id' => '986172', 'cat_name' => 'Category
+2', }
};

# 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();
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