How do you flatten a hash of key,value pairs?
sub flatten{
    my($ref,$delim,$item_delim,$array,$str) = @_;

    die("Required Hash Reference") unless isHash($ref);

    $delim = $delim ? $delim  :'_';

      #dump into array hash vals #simplified
      if(!$item_delim){
        @{$array} = %{$ref};
      }else{
        my($keys,$values);

        $keys = getKeys($ref);
        $values = getValues($ref);

        #item strings
        if($#$keys > 0 && $#$values > 0){
          #fix for issue where value[n] is empty
          @{$array}= map{ (defined $$values[ $_ ]) ? $$keys[ $_ ].$item_delim.$$values[ $_ ] : $$keys[ $_ ].$item_delim } 0 .. int($#$keys);
        }else{
         log "No Values to flatten";
         return '';
        }
      }

    $str = join($delim,@{$array});
    return $str;
  }

$HASH => {

 key1 => 'val1',
 key2 => 'val2',
 key3 => 'val3',

}

sub flatten {
  my ($hash, $delim, $item_delim) = @_;

  $delim //= '&',
  $item_delim //= '=';

  return join $delim, map { "$_$item_delim$hash->{$_}" } keys %$hash;
}

use URI;

my $uri = URI->new("", "http"); # We don't actually care about the path...
$uri->query_form(%params);
my $query_string = $uri->query;

my $hashref = {
  key1 => 'val1',
  key2 => 'val2',
  key3 => 'val3',
};

sub encode {
  my $str = shift;
  $str =~ s/([^A-Za-z0-9./_-])/sprintf("%%%02X", ord($1))/seg;
  return $str;
}

my $str = join '&' => map { encode($_).'='.encode($hashref->{$_}) } grep { defined $hashref->{$_} } keys %$hashref;

key2=val2&key1=val1&key3=val3

use URI::Escape;
my $str=join '&',map {uri_escape($_).'='.uri_escape($QUERY_STRING->{$_})} grep {defined $QUERY_STRING->{$_}} keys %$QUERY_STRING;