Maybe use Time::Piece to generate the added days.

#!perl
use strict;
use Time::Piece;
use Time::Seconds 'ONE_DAY';

my $process_date  = '20161002';
my $t = Time::Piece->strptime($process_date,"%Y%m%d");
#my $t = Time::Piece->new(); # today
print "Process date is ".$t->ymd('')."\n";

my $data_date  = '20160930';
my $d = Time::Piece->strptime($data_date,"%Y%m%d");
print "Date in data is ".$d->ymd('')."\n";

$d += ONE_DAY;
while ($d <= $t){
  print "   Add date ".$d->ymd('')."\n";
  $d += ONE_DAY;
}
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