#!/usr/bin/perl

use strict;
use warnings;
use List::Util qw( uniq );

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

my $string = join ',', find( keys %$data );
print "$string\n";

sub find
  {
  uniq map {
    my $ref = $data->{$_};
    $ref ? ( find( $ref->{cat_parent_id} ), $ref->{cat_name} ) : ()
    } @_;
  }