use strict;
open INFILE,"infile.txt";
open OFILE,">outfile.txt";
my $total_os=0;
while(<INFILE>){
  my $tmp=$_;
  $str=~s/(\s+)/osmarker(pos($str),$1)/xeg;

  # a bunch of regular expressions

  $total_os+=length($_);
  print OFILE $str;
}

sub osmarker{
  my $os=shift;
  my $spaces=shift;
  $os+=length($spaces)+total_os;
  return $spaces."<OS=$os>";
}

##</code><code>##

my $space=qr/(?:<OS=\d+>|\s)/;