use strict;
use warnings; 

my @keys = qw(A B C D E);     
open my $fh, '<', $filename_from_hdfs
   or die "$!";

my %mapping;
while ( my $line = readline($fh) ) {
   chomp($line);
   my ($key, $value) = split /,/, $line; 

   $mapping{$key} = $value; 
}

# zero out keys with no value

$mapping{$_} //= 0 foreach @keys; 

print join(',', map { "$_=$mapping{$_}" } sort keys %mapping), "\n";

my %result;

# assign zero to all keys

for my $key ('A' .. 'E')
{
 $result{$key} = 0;
}

# open file, read each line one by one

# Split each read line from file in ($key, $value).

result{$key} = $value;

# After finishing reading the file, traverse %result and output key, values