So what is the problem? Just set up a server to listen on (presume port 389), forward it to the real LDAP server, get the response, modify it and pass it back. Probably a job for POE::Component::Proxy::TCP but you could use IO::Socket and IO::Select if you want to go lower level. There is always Socket if you really want to get your hands dirty. With POE your code might look like:

use POE qw(Component::Proxy::TCP);
POE::Component::Proxy::TCP->new(
   Alias             => "LDAPProxyServerSessionAlias",
   Port              => $local_server_port,
   OrigPort          => $remote_server_port,
   OrigAddress       => $remote_server_host,
   DataFromClient    => \&data_from_client_handler,
   DataFromServer    => \&data_from_server_handler,
);

sub data_from_client_handler {...}
sub data_from_server_handler {...}
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