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"],
)