my %diff;
my %new;

open F, "< $old_file" or die "read $old_file: $!";
while (<F>) {
  chomp;
  my( $cid, $date, $avs ) = split /::/;
  $diff{"$cid::$date"}{$avs}++;
}
close F;

open F, "< $new_file" or die "read $new_file: $!";
while (<F>) {
  my( $cid, $date, $avs ) = split /::/;
  my $key = join '::', $cid, $date;
  $diff{$key}{$avs}++;
  $new{$key} = $_;
}
close F;

while ( my( $key, $val_hr ) = each %diff ) {
  keys %$val_hr > 1 and $data .= $new{$key};
}