#!/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
Outputs:
{
900000002 => "2021-03-10T13:58:50.000Z",
900000007 => "2021-03-10T17:11:24.000Z",
}