#!/usr/bin/perl
while(<DATA>){

  if (/^(\[\w+\])/) { # New block starts
    $line =~ s/\<$/<p>/
     if $f eq 'NEW'; # Replace the < by <p> for NEW blocks only
    $f = $1;
  }

 defined($line) and print $line; # Print the last line
 $line = $_; # keep for next run # Save this line
}
print $line; # Print last line

__DATA__
[NEW]
 Dear Carolyn: My husband and I divorced after 42 years. It didn't tak
+e long
for him to find a replacement, but I was happy for him at the time.<
them and let my grandchildren dig through her purse as if she were the
+ir
grandmother.<
[OLD]