As per pme, I made the following changes but nothing worked.

#!/usr/bin/perl

use strict;
use warnings;
use XML::CSV;
use Text::CSV;
use Time::Local;

my @d;
my %ha;
my $file = 'dDos_flows.csv';
my $file1 = 'dDos_flows1.csv';
open(my $fh,'<:encoding(UTF-8)',$file);
open(my $fh1,'>:encoding(UTF-8)',$file1);
while(my $f=<$fh>)
{
chomp $f;
my @data=split ",",$f;
my $res=$data[2];
#print "$res \n";
my @res= split(' ', $res);
shift @res;

foreach my $t (@res) {
    print localtime($t) . "\n";
}
#my $conv=scalar(localtime($data[2]));
#print "$conv \n";
}
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output

Use of uninitialized value $res in split at D:\ddos.pl line 21, <$fh> 
+line 1.
Use of uninitialized value $res in split at D:\ddos.pl line 21, <$fh> 
+line 2.
Use of uninitialized value $res in split at D:\ddos.pl line 21, <$fh> 
+line 22.
Use of uninitialized value $res in split at D:\ddos.pl line 21, <$fh> 
+line 23.
Use of uninitialized value $res in split at D:\ddos.pl line 21, <$fh> 
+line 24.
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