Select Code to Download

my %h;

...
  $cmp{$_} = '' for values %{$h{$id}};
  print keys %cmp if scalar keys %cmp == 1;
}

while (<>) {
  my @fields = split ',';
...
for my $id (sort keys %h) {
    print keys %{$h{$id}} if scalar keys %{$h{$id}} == 1;
}

scalar keys %{$h{$_}} == 1 and print keys %{$h{$_}} for sort keys %h;

perl -ane '$h{$F[0]}{$_}=0;END{keys%{$h{$_}}==1&&print keys%{$h{$_}}fo
+r sort keys%h}' f1.txt f2.txt