To try and cover the overhead from the Net::LDAP::Entry objects I mocked this up:

use strict;
use warnings;
use Net::LDAP;
use Net::LDAP::Entry;

my @entrylist;

for (1..500000)
{
    my $entry = Net::LDAP::Entry->new();
    $entry->dn("cn=name$_,ou=test,dc=test,dc=uk");
    $entry->add(cn=>"name$_");
    push @entrylist,$entry;
    print "$_\n";
}
while (<STDIN>){}
[download]

which results in the process using 12MB on my (wintel) box. You might want to try it on linux.