Just some idea on how to handle this

sort primary file (100s of millions)
split file into ~ 10 milion record sub files
  named for the last key contained in each file

sort secondary file -> ssf1
   you may sort multiple secondary files (ssf2, ssf3 etc)

perl program 
   read directory containing primary files by name
   create an array containing the file names (@pfn)(last key in the ar
+ray)

   open ssf1
        ssf2
        ssf3
    ssf4

     $ssf1_record=" ";  # or a value lower than lowest value
     $ssf2_record=" ";  # or a value lower than lowest value
     $ssf3_record=" ";  # or a value lower than lowest value
     $ssf4_record=" ";  # or a value lower than lowest value

   foreach $pf (@pfn){
     open primary file ($pf)
    read $pf records into a hash ($pfrh)

     while ($ssf1_record < $pf){
    compare to hash
    if found{
      action
    }
        read $ssf1 record;
     }

     while ($ssf2_record < $pf){
    compare to hash
    if found{
      action
    }
        read $ssf2 record;
     }
  }
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