$VAR1 = {
          '10' => 'Test1,Test2,Test3,Test4,Test5,Test6',
          '20' => 'Test4'
        };

##</code><code>##

$VAR1 = {
          '10' => 'Test1,Test3,Test5,Test6'
        };


##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

my %new_reports = (
		     10 => 'Test1,Test2,Test3,Test4,Test5,Test6',
		     20 => 'Test4'
		);

print Dumper(\%new_reports);

my @key_to_search = qw(10 20);
my @element_to_delete = qw(Test2 Test4);

foreach (@key_to_search) {
			if($new_reports{$_}) {
				my @sel_reports = split(/,/,$new_reports{$_});
				map{
					my $element_to_remove = $_;
					@sel_reports = grep { $_ ne $element_to_remove } @sel_reports;
				 }@element_to_delete;
				if(scalar(@sel_reports) > 0) {
					$new_reports{$_} = join(",",@sel_reports);
				} else {
					delete $new_reports{$_};
				}
			}
}


print Dumper(\%new_reports);