Another approach is to not modify your DB with the missing day values, but rather implement a simple "lookup" or "search" function that returns the null values if a particular day value does not exist. This is one of many possibilities:

use strict;
use warnings;


my $nullRecord_ref = {'Balance' => '-', 'payment' => '-', 'Total' => '
+-' };

my %hash_of_days = 
(
   'day1' => {'Balance' => '183.57', 'payment' => 0, 'Total' => 0, },
   'day4' => {'Balance' => '12.86',  'payment' => 0, 'Total' => 0, },
);

foreach my $day (1..5,123)
{
   my %result = searchDB (\%hash_of_days, $day);
   print "Day = $day:\n";
   print "  ",$_ => "  ".$result{$_},"\n" foreach (keys %result);
}

sub searchDB
{
   my($DBref, $day) = @_;
   my %result;
   
   %result = %$nullRecord_ref;
   %result = %{$DBref->{"day$day"}} if exists $DBref->{"day$day"};
   return %result;
}

__END__
Prints:
Day = 1:
  Balance  183.57
  payment  0
  Total  0
Day = 2:
  payment  -
  Total  -
  Balance  -
Day = 3:
  Total  -
  payment  -
  Balance  -
Day = 4:
  payment  0
  Total  0
  Balance  12.86
Day = 5:
  Balance  -
  payment  -
  Total  -
Day = 123:
  Total  -
  payment  -
  Balance  -
[download]