# "Prime the pump"
my $rec1 = <FILE1>;
my $rec2 = <FILE2>;

# Keep looping as long as either file has records
while (!eof(FILE1) or !eof(FILE2)) {

   # Figure out what keys you have
   my $key1 = get_key_1($rec1);
   my $key2 = get_key_2($rec2);

   if ($key1 eq $key2) {
      # They're the same, so create an output record, and read
      # next record from file2
      print build_record($rec1, $rec2);
      $rec2 = <FILE2>;
   }
   elsif ($key1 lt $key2) {
      # First file has a key we don't need, just ignore
      # it and read the next record
      $rec1 = <FILE1>;
   }
   else {
      # Hmmm ... first file seemed to skip the key we need.
      # print a partial record and advance to next file2 record
      print partial_record($rec2);
      $rec2 = <FILE2>;
   }
}