#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11129526
use warnings;
use List::Util qw( maxstr );

my %last;
/^(9\d+).*?(\S+T\S+)/ and $last{$1} = maxstr $last{$1} // '', $2
  while <DATA>;

use Data::Dump 'dd'; dd \%last;

__DATA__
@array1            @array3             @array2    @arra3
900000001    900000001    Network1    Login
900000002    900000002    Network1    2019-11-07T20:31:10.000Z
900000002    900000002    Network1    2021-02-19T13:47:25.000Z
900000002    900000002    Network1    2021-03-10T13:58:50.000Z
900000003    NOT Registered    No Network1    NOT Registered
900000004    900000004    Network1    Login
900000005    900000005    Network1    Login
900000006    900000006    Network1    Login2
900000007    900000007    Network1    2021-03-10T17:11:24.000Z
900000007    900000007    Network1    2021-02-18T08:34:58.000Z
900000007    900000007    Network1    2021-01-25T17:19:44.000Z
[download]

Outputs:

{
  900000002 => "2021-03-10T13:58:50.000Z",
  900000007 => "2021-03-10T17:11:24.000Z",
}
[download]