With what part of the text do you have problems?

Next, we can translate the important parts into Perl:

    $db->run_command( [ validate => "foo" ] );
[download]

As this is a project for your studies, I can only recommend talking to your mentor about advice regarding the use of MongoDB.

Finally I completed like this

#!/usr/bin/perl

use warnings;
use strict;

use MongoDB;
use Time::Local;

my $date=$ARGV[0];
$date or $date=`date --date='90 day ago'  +%d-%m-%Y`;
chomp $date;
print "Deleting before this date: ",$date,"\n";

my $db_host="***.**.**.***"; #For security reasons stared my ip
my $db_name="ravi";
my $client     = MongoDB::MongoClient->new(host => $db_host, port => 2
+7017);
my $db   = $client->get_database( $db_name );
my $data = $db->get_collection('collection')->find();

while (my $row = $data->next)
{
    my $docDate   = $row->{'Date'}; 
    my $queryDate = "14-06-2015";                #substr("okay", 1,2)
    my $timestamp = ":00:00:00";
    $docDate = $docDate.$timestamp;
    my ($mday,$mon,$year,$hour,$min,$sec) = split(/[\s.:-]+/, $docDate
+);
    my $docDateMillis = timelocal($sec,$min,$hour,$mday,$mon-1,$year);

    $queryDate = $queryDate.$timestamp;
    ($mday,$mon,$year,$hour,$min,$sec) = split(/[\s.:-]+/, $queryDate)
+;
    my $queryDateMillis = timelocal($sec,$min,$hour,$mday,$mon-1,$year
+);
    
    if($docDateMillis < $queryDateMillis) 
    {
        my $id = $row->{'_id'};
        my $removed=$db->get_collection('collection')->remove({'_id' =
+> $id});
        print $id,":::Removed\n";
    } 
}
[download]