my %data;
my @fieldlist1 = split(/,/,<DATAONE>);
while(<DATAONE>) {
 my ($key,@rest) = split(/,/,$_);
 push @{$data{$key}}, @rest;
}
my @fieldlist2 = split(/,/,<DATATWO>);
shift @fieldlist2; # throw away the first field b/c it is the id

while( <DATATWO>) {
 my ($key,@rest) = split(/,/,$_);

 #if you want to JOIN these two files, 
 # and skip records where there is not <DATAONE>
 next unless $data{$key};

 push @{$data{$key}},@rest;
}

my @fields = (@fieldlist1,@fieldlist2);
# assuming the PK is numeric
print join(',',@fields),"\n";
foreach my $id ( sort { $a <=> $b } keys %data ) {
  # if you wanted to skip the lines where there 
  # was data in ONE but not in TWO
  # you need a count of the number of fields you expect
  # which is handily avaialable in @fields -1 (ignoring id)
   next unless scalar @{$data{$id}} == (scalar @fields -1);
 print join(',', $id, @{$data{$id}}),"\n";
}