my %seen;
while (<>) {
  chomp;
  my @fields = split('\|', $_);
  if (my $record = $seen{$fields[0]}) {
    ... append @fields to $record...
  } else {
    $seen{$fields[0]} = ...some data structure...
  }
}
for my $k (keys %seen) {
  my $record = $seen{$k};
  # process $record
}

##</code><code>##

my %seen;
$/ = "\n\n";
while (<>) {
  chomp;
  my ($idline, @letters) = split(/\n/, $_);
  my @fields = split('\|', $_);
  my $record = $seen{$fields[0]};
  unless ($record) {
    $seen{$fields[0]} = $record
       = { idline => $idline, letters => [ @letters ] };
  } else {
    push(@{$record->{letters}}, @letters);
  }
}
for keys (%seen) {
  print join("\n", $seen->{$_}->{idline}, @{$seen->{$_}->{letters}}, "\n"), "\n";
}