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";
}
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