use strict;
use v5.10;

my $hash = {
  '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_name' => 'Category2',
    'cat_parent_id' => '986172'
  }
};

sub rec {
  my ($hash, $parent) = @_;
  my @str;
  foreach my $k (keys %{ $hash } ) {
    if ($hash->{$k}->{cat_parent_id} eq $parent) {
      push @str, $hash->{$k}->{cat_name};
      push @str, rec($hash, $k);
      last;
    }
  }
  return @str;
}

my @arr = rec($hash, '');

say join(',', @arr);