use warnings;
use strict;
use Data::Dump;
use Text::CSV; # also install Text::CSV_XS for speed

my %data;

my $csv = Text::CSV->new({ binary=>1, auto_diag=>2,
	sep_char=>"|", allow_whitespace=>1 });
my $hdr = $csv->getline(*DATA);
while ( my $row = $csv->getline(*DATA) ) {
	push @{ $data{ $row->[0] } }, $row->[1];
}
$csv->eof or $csv->error_diag;

dd \%data;

__DATA__
ColA | ColB |
     500001| Network1|      
     500001| Network2|
     500002| Network2|
     500003| Network1|      
     500003| Network3|