Let me repeat myself:

Does the site work when you use the "normal" WWW::Mechanize instead?

By this, I mean to remove the mentions of WWW::Mechanize::GZip and replace them by WWW::Mechanize.

WWW::Mechanize (and likely, WWW::Mechanize::GZip) are derived from LWP::UserAgent and thus the approach outlined in LWP::Debug will work for them to dump the outgoing HTTP requests and the responses you get.

This is one side of the comparison. The other side, you can get for example by using the Mozilla HTTP Live Headers extension.

Compare the output of your program (resp. the dumped headers) with the headers you capture when manually navigating the site. Change your script until there are no more differences.

#!/usr/bin/perl
use WWW::Mechanize::GZip;
`cat /dev/null >  /var/www/html/r.out`;
$m=`date --date="0 days ago" +'%-m'`;
$m=~ s/\n//g;
$d=`date --date="0 days ago" +'%-d'`;
$d=~ s/\n//g;
$y=`date --date="0 days ago" +'%-Y'`;
$y=~ s/\n//g;
$start=$m."/".$d."/".$y;
#$start=trim($start);
my $mech = WWW::Mechanize::GZip->new();
$response = $mech->post("http://domain.com/affiliates/login.ashx?tp=1"
+,[u=>'xxxxxx',p=>'yyyyyyy']);
$response = $mech->post("http://domain.com/affiliates/Extjs.ashx?s=sub
+affsummary",[start_date=>'8/7/2016',end_date=>'8/7/2016',csv=>'1']);
$mech->save_content("/var/www/html/uv__$y$m$d.csv");
print $response->content();
[download]

//this my script. Kindly guide me

Replace  WWW::Mechanize::GZip with  WWW::Mechanize and see what happens

Read LWP::Debug and do what it says to your $mech object, and see what output you get (  $mech->->add_handler ...)

Then fireup firefox browser (firefox.exe or ...) install livehttpheaders addon, and compare what the http that livehttpheaders captures to the output that LWP::Debug generates

Then simply tell your program to send same/similar headers as https://metacpan.org/pod/WWW%3A%3AMechanize%3A%3AFAQ#I-thought-Mech-was-supposed-to-work-like-a-web-browser explains