Win8 Strawberry 5.8.9.5 (32)  Fri 05/14/2021  4:48:55
C:\@Work\Perl\monks\workInProgress12
>perl

use strict;
use warnings;
use autodie;

use Data::Dump qw(dd);

my $data = <<'EOD';
ColA | ColB |
     500001| Network1|
     500003| Network1|
     500002| Network2|
     500001| Network2|
     500003| Network3|
EOD

open my $fh, '<', \$data;

<$fh>;  # ignore headings record/line

my %hash4;

while (<$fh>) {

    my ($k, $v) =
        map { (my $r = $_) =~ s{ \A \s+ | \s+ \z }{}xmsg;  $r; }
        split /\s*\|\s*/  #splits $_ implicitly
        ;
  # print "\$k '$k'  \$v '$v' \n";  # for debug

    push @{ $hash4{$k} }, $v;

    }

dd \%hash4;

close $fh;

^Z
{
  500001 => ["Network1", "Network2"],
  500002 => ["Network2"],
  500003 => ["Network1", "Network3"],
}