perl -MData::Dump -F"\|\s?" -lane "$.==1?next:push@{$h{eval$F[0]=~tr/\s//r}},$F[1]}{dd%h" columndata.txt

(
  500002,
  ["Network2"],
  500003,
  ["Network1", "Network3"],
  500001,
  ["Network1", "Network2"],
)

##</code><code>##

perl -MO=Deparse -MData::Dump -F"\|\s?" -lane "$.==1?next:push@{$h{eval$F[0]=~tr/\s//r}},$F[1]}{dd%h" columndata.txt

BEGIN { $/ = "\n"; $\ = "\n"; }
use Data::Dump;
LINE: while (defined($_ = readline ARGV)) {
    chomp $_;
    our @F = split(/\|\s?/, $_, 0);
    $. == 1 ? next : push(@{$h{eval $F[0] =~ tr/s//r};}, $F[1]);
}
{
    dd(%h);
}
-e syntax OK