if particular (exact) tags match a particular file to replace the content, then you are not at all dealing with regex, but instead a simple string comparison (for the tags that is). something along the lines of the following should just about do it, if i'm understanding your question correctly.

while($line=<>)
{
  chomp $line;
  if($line=~m/<REANALYZED>/)
  {
    print "$line\n";
    system("cat new_content_for_reanalyzed_tag_file");
    do
    {
      $line=<>;
      chomp $line;
    } until($line=~m/<\/REANALYZED>/);
    print "$line\n";
  }
  elsif($line=~m/<ANOTHERTAG>/)
  {
    print "$line\n";
    system("cat another_file_for_this_tag");
    .
    .
    .
    etc.
  }
  else
  {
    print "$line\n";
}
[download]

I'm sure you can make that much simpler by using some perlisms and make some subs out of re-usable code, but that's the main jist of it. Use redirection to feed this program with your input file, and redirect standard output as you need. E.g. perl this_script.pl <orig_file >new_file