`Making Two Passes`

This program has an entire preprocessing step:

      3 $input1 = 'chaintest.scan';
      4 $output= "OUT";
      5 $output2= "OUT.out";
      6 
      7 open (INFILE,"$input1")||die "cannot open $input1";
      8 open(OUTFILE,">$output")||die "canoot\n";
      9 open(OUTFILE2,">$output2")||die "canoot\n";
     10 
     11 #######################################################
     12         &preprocess;
     13 open (INFILE2,"$output")||die "cannot open $input1";

What is it doing?